drivers: clk: st: Support for enable/disable in Clockgen PLLs

The patch adds support for enable/disable of the Clockgen PLLs.
clkgen_pll_enable/clkgen_pll_disable added as generic function for all PLLs.

Signed-off-by: Pankaj Dev <pankaj.dev@st.com>
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>