clk: fsl-flexspi: new driver

Add support for the FlexSPI clock on Freescale Layerscape SoCs. The
clock is a simple divider based one and is located inside the device
configuration space (DCFG).

This will allow switching the SCK frequencies for the FlexSPI interface
on the LS1028A and the LX2160A.

Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20201108185113.31377-8-michael@walle.cc
[sboyd@kernel.org: Drop modalias, add module table]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>