ARM: 9439/1: arm32: simplify ARM_MMU_KEEP usage

All current users need to add a KEEP() around the argument so the value is
actually kept, which doesn't feel very natural and is prone to upcoming bugs as
the name suggests that this macro alone already keeps things. Move that directly
into the definition.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>