clk: clk-gate: Create clk_gate_endisable()

This patch tries to remove duplicate code for clk_gate clocks. This creates
another routine clk_gate_endisable() which will take care of enable/disable
clock with knowledge of CLK_GATE_SET_TO_DISABLE flag.

It works on following logic:

For enabling clock, enable = 1
	set2dis = 1	-> clear bit	-> set = 0
	set2dis = 0	-> set bit	-> set = 1

For disabling clock, enable = 0
	set2dis = 1	-> set bit	-> set = 1
	set2dis = 0	-> clear bit	-> set = 0

So, result is always: enable xor set2dis.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>