hw/arm_boot.c: Make ram_size a uint64_t

hw/arm_boot.c: Make ram_size a uint64_t

Make the RAM size in arm_boot_info a uint64_t so it can express
the larger RAM sizes that may be seen in LPAE systems.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>