parisc: Allow CONFIG_64BIT with ARCH=parisc

The previous patch triggered a build failure for the debian kernel,
which has CONFIG_64BIT enabled, uses the CROSS_COMPILER environment
variable and uses ARCH=parisc to configure the kernel for 64-bit
support.

This patch weakens the previous patch while keeping the recommended way
to configure the kernel with:
    ARCH=parisc     -> build 32-bit kernel
    ARCH=parisc64   -> build 64-bit kernel
while adding the possibility for debian to configure a 64-bit kernel
even if ARCH=parisc is set (PA8X00 CPU has to be selected and
CONFIG_64BIT needs to be enabled).

The downside of this patch is, that we now have a small window open
again where people may get it wrong: if they enable CONFIG_64BIT and try
to compile with a 32-bit compiler.

Fixes: 3dcfb729b5 ("parisc: Make CONFIG_64BIT available for ARCH=parisc64 only")
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org> # 5.15+

arch/parisc/Kconfig

@@ -224,8 +224,18 @@ config MLONGCALLS
 	  Enabling this option will probably slow down your kernel.
 
 config 64BIT
-	def_bool "$(ARCH)" = "parisc64"
+	def_bool y if "$(ARCH)" = "parisc64"
+	bool "64-bit kernel" if "$(ARCH)" = "parisc"
 	depends on PA8X00
+	help
+	  Enable this if you want to support 64bit kernel on PA-RISC platform.
+
+	  At the moment, only people willing to use more than 2GB of RAM,
+	  or having a 64bit-only capable PA-RISC machine should say Y here.
+
+	  Since there is no 64bit userland on PA-RISC, there is no point to
+	  enable this option otherwise. The 64bit kernel is significantly bigger
+	  and slower than the 32bit one.
 
 choice
 	prompt "Kernel page size"