ARM: 6737/1: SPEAr: formalized timer support

Move platform specific timer initialization code is moved into platform
specific files.

Reviewed-by: Jamie Iles <jamie@jamieiles.com>
Reviewed-by: Stanley Miao <stanley.miao@windriver.com>
Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Rajeev Kumar <rajeev-dlh.kumar@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>