Replace scu_base_addr() with a static shmobile_scu_base
variable and introduce R8A7779_SCU_BASE.

Later in the series the shmobile_scu_base variable will be
made into a global variable so this is preparation only.
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>