soc: imx: imx8mp-blk-ctrl: expose high performance PLL clock

Expose the high performance PLL as a regular Linux clock, so the
PCIe PHY can use it when there is no external refclock provided.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-by: Lukas F. Hartmann <lukas@mntre.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>