npcx7: fix RAM powerdown regression
npcx7: fix RAM powerdown regression

chromium:1518660 introduced a subtle change in which RAM bank
power-down bits are set.  this change reverts to the original bit
pattern and masks the reserved bits as suggested by nuvoton.

BUG=b:141715856
BRANCH=none
TEST=boots on atlas.

Change-Id: Idfbedd73fd46aa639f12c9991911d96f15739174
Signed-off-by: Caveh Jalali <08e24a66aefda7a55ca7d73f8903e0fb16eec763@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1833153
Reviewed-by: Caveh Jalali <08e24a66aefda7a55ca7d73f8903e0fb16eec763@google.com>
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec