sunxi: SPL: store RAM size in gd

The sunxi SPL was holding the detected RAM size in some local variable
only, so it wasn't accessible for other functions.
Store the value in gd->ram_size instead, so it can be used later on.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>