ARM: shmobile: Common shmobile_scu_base in headsmp-scu.S

Update the code in headsmp-scu.S to use a global
shmobile_scu_base variable both for convenient SCU
base address storage and for the early SCU setup
code in shmobile_secondary_vector_scu.

With this patch applied r8a7779, sh73a0 and EMEV2
all make use of the global shmobile_scu_base
variable. However only sh73a0 makes use of the SCU
bring up code in shmobile_secondary_vector_scu.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>