imx: mx8m: add clock driver

Add clock driver to support i.MX8M.

There are two kind PLLs, FRAC pll and SSCG pll. ROM already
configured SYS PLL1/2, we only need to configure the output.
ocotp/i2c/pll decoding and configuration/usdhc/lcdif/dram pll/
enet clock are configured in the code.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>