ARM: 6932/1: SPEAr3xx: Replace defconfigs with single unified defconfig

We only need one defconfig for SPEAr3xx now since we can build all
three boards into one kernel.

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>