phy: ralink: phy-mt7621-pci: use kernel clock APIS

MT7621 SoC clock driver has already mainlined in
'commit 48df7a26

 ("clk: ralink: add clock driver for mt7621 SoC")'
This allow us to properly use kernel clock apis to get
the clock frequency needed for the phy configuration
instead of use custom architecture code to do the same.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210508070930.5290-4-sergio.paracuellos@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>