ARM: 6020/1: ST SPEAr: Adding gpio pad multiplexing support

GPIO Pads in spear platform are are multiplexed in various machines.
This patch adds support for this pad multiplexing.

Reviewed-by: Linus Walleij <linux.walleij@stericsson.com>
Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>