drivers: soc: sunxi: Fix mask generation for SRAM mapping

GENMASK is inclusive on both ends, therefor one has to be
subtracted from the width.
Also fixes the mask for debug output.

Signed-off-by: Jens Kuske <jenskuske@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>