The ipq806x_gmac_probe() function enables the PTP clock but not the
appropriate interface clocks. This means that if the bootloader hasn't
done so attempting to bring up the interface will fail with an error
like:

[   59.028131] ipq806x-gmac-dwmac 37600000.ethernet: Failed to reset the dma
[   59.028196] ipq806x-gmac-dwmac 37600000.ethernet eth1: stmmac_hw_setup: DMA engine initialization failed
[   59.034056] ipq806x-gmac-dwmac 37600000.ethernet eth1: stmmac_open: Hw setup failed

This patch, a slightly cleaned up version of one posted by Sergey
Sergeev in:

https://forum.openwrt.org/t/support-for-mikrotik-rb3011uias-rm/4064/257

correctly enables the clock; we have already configured the source just
before this.

Tested on a MikroTik RB3011.
Signed-off-by: Jonathan McDowell <noodles@earth.li>
Signed-off-by: David S. Miller <davem@davemloft.net>