[PARISC] Document some register usages in assembly files

Document clobbers and args in entry.S and syscall.S.

entry.S: Add comment to indicate that cr27 may recycle and EDEADLOCK
detection is not 100% correct. Since this is only enabled when using
ENABLE_LWS_DEBUG, the user is warned by the comment.
Signed-off-by: Carlos O'Donell <carlos@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>

arch/parisc/kernel/entry.S

@@ -1846,6 +1846,7 @@ sys_clone_wrapper:
 	ldo	-16(%r30),%r29		/* Reference param save area */
 #endif
 
+	/* WARNING - Clobbers r19 and r21, userspace must save these! */
 	STREG	%r2,PT_GR19(%r1)	/* save for child */
 	STREG	%r30,PT_GR21(%r1)
 	BL	sys_clone,%r2

arch/parisc/kernel/syscall.S

@@ -164,7 +164,7 @@ linux_gateway_entry:
 #endif
 	STREG	%r2,  TASK_PT_GR30(%r1)		/* ... and save it */
 	
-	STREG	%r20, TASK_PT_GR20(%r1)
+	STREG	%r20, TASK_PT_GR20(%r1)		/* Syscall number */
 	STREG	%r21, TASK_PT_GR21(%r1)
 	STREG	%r22, TASK_PT_GR22(%r1)
 	STREG	%r23, TASK_PT_GR23(%r1)		/* 4th argument */
@@ -527,6 +527,7 @@ lws_compare_and_swap:
 		We *must* giveup this call and fail.
 	*/
 	ldw	4(%sr2,%r20), %r28			/* Load thread register */
+	/* WARNING: If cr27 cycles to the same value we have problems */
 	mfctl	%cr27, %r21				/* Get current thread register */
 	cmpb,<>,n	%r21, %r28, cas_lock		/* Called recursive? */
 	b	lws_exit				/* Return error! */