docs: sh: convert register-banks.txt to ReST

- Add a SPDX header;
- Adjust document title to follow ReST style;
- Add blank lines to make ReST markup happy
- Add it to sh/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/adf117cf1edd7f43cb839ff2800f4315dfbcce13.1592203650.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sh/index.rst

@@ -8,6 +8,7 @@ SuperH Interfaces Guide
     :maxdepth: 1
 
     new-machine
+    register-banks
 
 Memory Management
 =================

Documentation/sh/register-banks.rst

@@ -1,3 +1,6 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================================
 Notes on register bank usage in the kernel
 ==========================================
 
@@ -23,11 +26,15 @@ Presently the kernel uses several of these registers.
 
 	- r0_bank, r1_bank (referenced as k0 and k1, used for scratch
 	  registers when doing exception handling).
+
 	- r2_bank (used to track the EXPEVT/INTEVT code)
+
 		- Used by do_IRQ() and friends for doing irq mapping based off
 		  of the interrupt exception vector jump table offset
+
 	- r6_bank (global interrupt mask)
+
 		- The SR.IMASK interrupt handler makes use of this to set the
 		  interrupt priority level (used by local_irq_enable())
-	- r7_bank (current)
 
+	- r7_bank (current)

arch/sh/Kconfig.cpu

@@ -85,7 +85,7 @@ config CPU_HAS_SR_RB
 	  that are lacking this bit must have another method in place for
 	  accomplishing what is taken care of by the banked registers.
 
-	  See <file:Documentation/sh/register-banks.txt> for further
+	  See <file:Documentation/sh/register-banks.rst> for further
 	  information on SR.RB and register banking in the kernel in general.
 
 config CPU_HAS_PTEAEX