armv8: cavium: Get DRAM size from ATF

Change the dram_init() function on ThunderX to query ATF services for
the real installed DRAM size

Signed-off-by: Sergey Temerkhanov <s.temerkhanov@gmail.com>
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>