[ARM] 5471/2: U300 GPIO and PADMUX support

This adds GPIO and PADMUX headers and implementation for
the U300 platform. This is an implementation in isolation
that depend on later patches in this series to plug into
the framework.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>