Fix relocation of __PERCPU_BAKERY_LOCK_SIZE__

Fix relocation of __PERCPU_BAKERY_LOCK_SIZE__

When a platform port does not define PLAT_PERCPU_BAKERY_LOCK_SIZE, the total
memory that should be allocated per-cpu to accommodate all bakery locks is
calculated by the linker in bl31.ld.S. The linker stores this value in the
__PERCPU_BAKERY_LOCK_SIZE__ linker symbol. The runtime value of this symbol is
different from the link time value as the symbol is relocated into the current
section (.bss). This patch fixes this issue by marking the symbol as ABSOLUTE
which allows it to retain its correct value even at runtime.

The description of PLAT_PERCPU_BAKERY_LOCK_SIZE in the porting-guide.md has been
made clearer as well.

Change-Id: Ia0cfd42f51deaf739d792297e60cad5c6e6e610b