ARM: shmobile: Fix base address readout in headsmp-scu.S

Rework the early SCU setup code in headsmp-scu.S to read
the base address in the same way as we use to fetch the
address of the invalidation function.

Reported-by: Bastian Hecht <hechtb@gmail.com>
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>