Move BL1 SMC FIDs macros out of platform layer

BL1 SMC function IDs are not platform-specific so move them to a new
generic header file, called bl1.h.

Change-Id: I621483f7737f8101e9f370343e1a45a731c31c3b
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>