docs: networking: device drivers: convert toshiba/spider_net.txt to ReST

- add SPDX header;
- adjust title markup;
- mark code blocks and literals as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/device_drivers/index.rst

@@ -50,6 +50,7 @@ Contents:
    ti/cpsw_switchdev
    ti/cpsw
    ti/tlan
+   toshiba/spider_net
 
 .. only::  subproject and html
 

Documentation/networking/device_drivers/toshiba/spider_net.rst

@@ -1,4 +1,6 @@
+.. SPDX-License-Identifier: GPL-2.0
 
+===========================
 The Spidernet Device Driver
 ===========================
 
@@ -78,7 +80,7 @@ GDACTDPA, tail and head pointers. It will also summarize the contents
 of the ring, starting at the tail pointer, and listing the status
 of the descrs that follow.
 
-A typical example of the output, for a nearly idle system, might be
+A typical example of the output, for a nearly idle system, might be::
 
     net eth1: Total number of descrs=256
     net eth1: Chain tail located at descr=20
@@ -101,7 +103,7 @@ The status x4... corresponds to "full" and status xa... corresponds
 to "empty". The actual value printed is RXCOMST_A.
 
 In the device driver source code, a different set of names are
-used for these same concepts, so that
+used for these same concepts, so that::
 
     "empty" == SPIDER_NET_DESCR_CARDOWNED == 0xa
     "full"  == SPIDER_NET_DESCR_FRAME_END == 0x4
@@ -137,7 +139,7 @@ while the hardware is waiting for a different set of descrs to become
 empty.
 
 A call to show_rx_chain() at this point indicates the nature of the
-problem. A typical print when the network is hung shows the following:
+problem. A typical print when the network is hung shows the following::
 
     net eth1: Spider RX RAM full, incoming packets might be discarded!
     net eth1: Total number of descrs=256
@@ -198,7 +200,3 @@ For large packets, this mechanism generates a relatively small number
 of interrupts, about 1K/sec. For smaller packets, this will drop to zero
 interrupts, as the hardware can empty the queue faster than the kernel
 can fill it.
-
-
- ======= END OF DOCUMENT ========
-

MAINTAINERS

@@ -15874,7 +15874,7 @@ SPIDERNET NETWORK DRIVER for CELL
 M:	Ishizaki Kou <kou.ishizaki@toshiba.co.jp>
 L:	netdev@vger.kernel.org
 S:	Supported
-F:	Documentation/networking/device_drivers/toshiba/spider_net.txt
+F:	Documentation/networking/device_drivers/toshiba/spider_net.rst
 F:	drivers/net/ethernet/toshiba/spider_net*
 
 SPMI SUBSYSTEM