ARM: shmobile: Remove romImage CONFIG_MEMORY_START

Instead of relying on CONFIG_MEMORY_START for memory
base address, let each romImage board header file
specify this information.

This is reworks code not to rely on CONFIG_MEMORY_START
which in turn is needed for ARCH_MULTIPLATFORM.

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