clk: imx: imx8qxp-lpcg: add parsing clocks from device tree

One LPCG controller supports up to 8 clock outputs while each of them
is fixed to 4 bits.  It supports only gating function with fixed bits.
So we can use the clk-indices to fetch the corresponding clock idx from
device tree. With this way, we can write a generic LPCG clock drivers.

This patch add that support to parse clocks from device tree.

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