Rationalise MMU and Page table related constants on ARM platforms

Rationalise MMU and Page table related constants on ARM platforms

`board_arm_def.h` contains multiple definitions of
`PLAT_ARM_MMAP_ENTRIES` and `MAX_XLAT_TABLES` that are optimised for
memory usage depending upon the chosen build configuration. To ease
maintenance of these constants, this patch replaces their multiple
definitions with a single set of definitions that will work on all ARM
platforms.

Platforms can override the defaults with optimal values by enabling the
`ARM_BOARD_OPTIMISE_MMAP` build option. An example has been provided in
the Juno ADP port.

Additionally, `PLAT_ARM_MMAP_ENTRIES` is increased by one to accomodate
future ARM platforms.

Change-Id: I5ba6490fdd1e118cc9cc2d988ad7e9c38492b6f0