The SAMA5D2 and SAM9X60 both feature generic clocks, which we already
support in barebox proper, but not in PBL. Add PBL support for setting
the parent and enabling them, so we may use it to enable the SDMMC
peripherals in first stage.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>