arm: Remove S bit from MMU section entry

Restore the old behavior of the MMU section entries configuration,
which is without the S-bit.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Simon Glass <sjg@chromium.org>