clk: Add support for power of two type dividers

Quite often dividers and the value programmed in the
register have a relation of 'power of two', something like
value	div
0	1
1	2
2	4
3	8...

Add support for such dividers as part of clk-divider.

The clk-divider flag 'CLK_DIVIDER_POWER_OF_TWO' should be used
to define such clocks.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>