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 <[e-mail]>
Link: https://lore.kernel.org/r/adf117[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/sh/index.rst

···
     :maxdepth: 1
 
     new-machine
+    register-banks
 
 Memory Management
 =================

Documentation/sh/register-banks.txt Documentation/sh/register-banks.rst

···
-	Notes on register bank usage in the kernel
-	==========================================
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================================
+Notes on register bank usage in the kernel
+==========================================
 
 Introduction
 ------------
···
 
 	- 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

···
 	  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