SPM: Declare explicit width based types in secure_partition_boot_info_t structure

The secure_partition_boot_info_t structure is used to communicate boot
parameters with the StandaloneMM code executing at S-EL0 through a
shared buffer. Certain data types used for members of this structure
are opaque with their size depending on the toolchain being used.

Declare the members of the structure with explicit width based data
types, which would maintain compatibility across toolchains.

Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>