drivers: net: cpts: Remove hardcoded clock name for CPTS

CPTS refclk name is hardcoded, which makes it fail in case of DRA7x
Remove the hardcoded clock name for CPTS refclk and get the same from DT.

Signed-off-by: George Cherian <george.cherian@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>