ARM: shmobile: Introduce shmobile_boot_size

Introduce shmobile_boot_size that can be used by
future SMP code to determine the size of the boot
code that needs to be copied to internal SRAM.

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