clk: imx: Add support for pllv3 enet clock

This code has been ported from Linux kernel v5.5.5 (tag) and has been
adjusted to U-Boot's DM.

It adds support for correct recognition of IMX_PLLV3_ENET flag in the
clk-pllv3.c driver.

Signed-off-by: Lukasz Majewski <lukma@denx.de>