SPM: Fix pointer to MP info in boot info struct

SPM: Fix pointer to MP info in boot info struct

The MP info struct is placed right after the boot info struct. However,
when calculating the address of the MP info, the size of the boot info
struct was being multiplied by the size of the MP boot info. This left
a big gap of empty space between the structs.

This didn't break any code because the boot info struct has a pointer to
the MP info struct. It was just wasting space.

Change-Id: I1668e3540d9173261968f6740623549000bd48db
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>