ARM: 6234/1: move SPEAr to use common mult/shift calculation code

Since the infrastructure for determining shift/mult values from a
clock rate and a minimum programmable rate is now in the kernel,
lets remove this custom code.

Cc: Viresh KUMAR <viresh.kumar@st.com>
Tested-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>