ARM: 6679/1: SPEAr: make clk API functions more generic

- Add a dummy clk_set_rate() function.  This is required for compilation
  of a few drivers.
- Make functions in plat-spear/clock.c more generic over all SPEAr
  platforms.
- Add div_factor in struct clk for clks with .recalc = follow_parent
- Change type of register pointers to void __iomem *

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