Readjust BL2 size after sharing Mbed TLS heap

After introducing the Mbed TLS shared heap optimisation, reducing BL2
size by 3 pages didn't leave enough space for growth. We give 1 page
back to maximum BL2 size.

Change-Id: I4f05432f00b923693160f69a4e4ec310a37a2b16
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>

include/plat/arm/board/common/board_arm_def.h

@@ -102,7 +102,7 @@
  * little space for growth.
  */
 #if TRUSTED_BOARD_BOOT
-# define PLAT_ARM_MAX_BL2_SIZE		0x1C000
+# define PLAT_ARM_MAX_BL2_SIZE		0x1D000
 #else
 # define PLAT_ARM_MAX_BL2_SIZE		0x11000
 #endif