clk: sifive: Add clock enable and disable ops

Add new functions "sifive_prci_clock_enable(), sifive_prci_clock_disable()
and sifive_clk_is_enabled()" to enable or disable the PRCI clock

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Tested-by: Zong Li <zong.li@sifive.com>
Link: https://lore.kernel.org/r/20201209094916.17383-6-zong.li@sifive.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>