arch: arm: clear r7 to match aarch64 smp implementation

add mov r7, #0 as aarch64 implementation uses wzr which returns 0
on read but r7 might have an undefined value after a reset.

Signed-off-by: Julien Racki <[email protected]>

Author: juickar

arch/arm/core/cortex_a_r/reset.S

@@ -238,6 +238,7 @@ EL1_Reset_Handler:
 
     /* suggest current core then release */
 1:  str r1, [r0, #BOOT_PARAM_MPID_OFFSET]
+    mov r7, #0
     strb r7, [r4, r2]
     dmb