ARM: mx6: Add PCI express clock configuration

Split the SATA clock enabling function and add PCI express clock
enabling function. The SATA clock enabling function starts up the
100MHz SATA reference PLL in ENET_PLL register, but the code can
be re-used to enable the 125MHz PCIe reference in ENET_PLL, so pull
this code into separate function. Moreover, add the PCIe clock
enabling code.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>