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
diff --git a/plat/arm/board/common/board_common.mk b/plat/arm/board/common/board_common.mk
index 9e0c848..da6beec 100644
--- a/plat/arm/board/common/board_common.mk
+++ b/plat/arm/board/common/board_common.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2015-2016, ARM Limited and Contributors. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are met:
@@ -54,3 +54,10 @@
     BL1_SOURCES		+=	plat/arm/board/common/board_arm_trusted_boot.c
     BL2_SOURCES		+=	plat/arm/board/common/board_arm_trusted_boot.c
 endif
+
+# This flag controls whether memory usage needs to be optimised
+ARM_BOARD_OPTIMISE_MMAP	?=	0
+
+# Process flags
+$(eval $(call assert_boolean,ARM_BOARD_OPTIMISE_MMAP))
+$(eval $(call add_define,ARM_BOARD_OPTIMISE_MMAP))