clk: keystone: Add new driver to handle syscon based clocks

On TI's AM654/J721e SoCs, certain clocks can be gated/ungated by setting
a single bit in SoC's System Control Module registers. Sometime more
than one clock control can be in the same register.

Add a driver to support such clocks using syscon framework. Driver
currently supports controlling EHRPWM's TimeBase clock(TBCLK) for AM654
SoC.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lkml.kernel.org/r/20200227053529.16479-3-vigneshr@ti.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>