[DOCS] Fix boot cpu check for ARMv7 and ARMv8 boot-wrapper

The MPIDR register of ARMv7 and ARMv8 contains more than 8 bits for
multiple affinity levels hence this patch fixes boot cpu checks in
reference boot-wrapper for ARM fast model and ARM foundation v8 model.

Signed-off-by: Anup Patel <anup@brainfault.org>