ARM: 6678/1: SPEAr: update padmux code

- compile padmux only for spear3xx
- padmux initialization code rearranged in evaluation board and machine
  files.

Reviewed-by: Stanley Miao <stanley.miao@windriver.com>
Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>