RealView: Allow access to the secure flash memory block on PB1176

This patch adds a Kconfig option for specifying whether Linux will only
be run in secure mode on the RealView PB1176 platform. Enabling it will
make the secure flash memory block (64MB @ 0x3c000000) available to
Linux.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>