FVP: Split MMU functions per exception level

Instead of having a single version of the MMU setup functions for all
bootloader images that can execute either in EL3 or in EL1, provide
separate functions for EL1 and EL3. Each bootloader image can then
call the appropriate version of these functions. The aim is to reduce
the amount of code compiled in each BL image by embedding only what's
needed (e.g. BL1 to embed only EL3 variants).

Also introduce generic macros IS_IN_ELX(), whose goal is to improve code
readability by providing a concise way to check whether we are running in
the expected exception level.