clk: imx: add lpcg clock support

The Low-Power Clock Gate (LPCG) modules contain a local programming
model to control the clock gates for the peripherals. An LPCG module
is used to locally gate the clocks for the associated peripheral.
And they're bedind the SCU clock.

Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>