hw/arm_boot.c: Consistently use ram_size from arm_boot_info struct

hw/arm_boot.c: Consistently use ram_size from arm_boot_info struct

Clean up the mix of getting the RAM size from the global ram_size
and from the ram_size field in the arm_boot_info structure, so
that we always use the structure field.

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