[ARM] Remove redundant smp_scu.h

Initially smp_scu was implemented as library providing access to
ARM SCU but now we have smp_scu implementing SMP OPS for ARM SCU.

Due to above smp_scu.h is no longer required hence this patch
removes it and updates smp_scu.c accordingly.

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