ARM: 6931/1: SPEAr3xx: Rework KConfig to allow all boards to be compiled in

Now that all three SPEAr3xx platforms can be built into one kernel,
rework KConfig to allow this. Move everything into one KConfig file
while we are here.

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