net: mvneta: Try to get named core clock first

Some platforms may provide more than one clk for the mvneta IP, for
example Marvell BG4CT provides one clk for the mac core, and one
clk for the AXI bus logic.

To support for more than one clock, we'll need to distinguish between
the clock by name. Change clock probing to first try to get "core"
clock before falling back to unnamed clock.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>