xen/arm32: head: Don't set r12 and update the documentation

Since commit 54c4ae18d1 ("xen/arm32: head: Rework and document
launch()"), the boot code is setting r12 but not read it.

So remove the two instructions setting r12 and update the documentation
to show r12 has no specific purpose.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

xen/arch/arm/arm32/head.S

@@ -79,7 +79,7 @@
  *   r9  - paddr(start)
  *   r10 - phys offset
  *   r11 - UART address
- *   r12 - is_secondary_cpu
+ *   r12 -
  *   r13 - SP
  *   r14 - LR
  *   r15 - PC
@@ -166,8 +166,6 @@ past_zImage:
 #endif
         PRINT("- Boot CPU booting -\r\n")
 
-        mov   r12, #0                /* r12 := is_secondary_cpu */
-
         bl    check_cpu_mode
         bl    cpu_init
         bl    create_page_tables
@@ -206,8 +204,6 @@ GLOBAL(init_secondary)
         adr   r9, start              /* r9  := paddr (start) */
         sub   r10, r9, r0            /* r10 := phys-offset */
 
-        mov   r12, #1                /* r12 := is_secondary_cpu */
-
         mrc   CP32(r1, MPIDR)
         bic   r7, r1, #(~MPIDR_HWID_MASK) /* Mask out flags to get CPU ID */