ARM: shmobile: r8a7779: Replace modify_scu_cpu_psr with scu_power_mode

We can remove the extra code of modify_scu_cpu_psr() and use the cleaner
generic ARM helper scu_power_mode(). As every CPU only deals with its
own power register and scu_power_mode() operates with 8-bit accesses,
we save the locking overhead too.

Signed-off-by: Bastian Hecht <hechtb@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>