xtensa: reorganize SR referencing

- reference SRs by names where possible, not by numbers;
- get rid of __stringify around SR names where possible;
- remove unneeded SR names from asm/regs.h;
- add SREG_ prefix to remaining SR names;

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/boot/boot-redboot/bootstrap.S

@@ -51,17 +51,17 @@ _start:
 	/* 'reset' window registers */
 
 	movi	a4, 1
-	wsr	a4, PS
+	wsr	a4, ps
 	rsync
 
-	rsr	a5, WINDOWBASE
+	rsr	a5, windowbase
 	ssl	a5
 	sll	a4, a4
-	wsr	a4, WINDOWSTART
+	wsr	a4, windowstart
 	rsync
 
 	movi	a4, 0x00040000
-	wsr	a4, PS
+	wsr	a4, ps
 	rsync
 
 	/* copy the loader to its address