rcar-gen3: control RPC hyper-flash access

RCAR_RPC_HYPERFLASH_LOCKED can be set to 0 as a build option if the
user needs to allow u-boot to reprogram the ATF firmware using a FIP
image (as a faster alternative of toggling numerous DIP switches on
the board and using ascii-xfer of srec files)

The code being controlled with this commit should only be re-enabled for
debugging (_never_ on a product release)

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez.ortiz@gmail.com>