if_cgem: Rewrite clock part

• pclk and hclk are mandatory so always try to get them. Don't make it fatal if it fails as some platform (like Zynq) don't have a proper clock driver.
• Always use pclk for the reference clock.
• Try to get all the possible clocks and enable them.

Reviewed-by: mhorne
Tested-by: Milan Obuch <bsd@dino.sk>
Differential Revision: https://reviews.freebsd.org/D41857
Sponsored by: Beckhoff Automation GmbH & Co. KG