ARMv8: make master CPU checking logic more clear

macro branch_if_master checks master CPU via (Aff3 & (Aff2:Aff1:Aff0))
it is simple but a little obscure.

fix by checking Affx fields within MPIDR_EL1 directly.

Signed-off-by: zijun_hu <zijun_hu@htc.com>