clk: ti: clkctrl: add new exported API for checking standby info

Standby status is provided for certain clkctrl clocks to see if the
given module has entered standby or not. This is mostly needed by
remoteproc code to see if the remoteproc has entered standby and the clock
can be turned off safely.

Signed-off-by: Tero Kristo <t-kristo@ti.com>