clk: qcom: clk-rcg2: Add support to force enable an RCG

In certain cases, it is desirable for an RCG to be turned on
explicitly instead of just relying on the enable signal from
its branch clock(s). Add support for this.

Change-Id: I2b0b81bddfe7cc7543c313e4ed13a332fb1a0fcb
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>
Signed-off-by: David Dai <daidavid1@codeaurora.org>