ar71xx: Fix gigabit switch support for Mikrotik RB951G-2HnD

ar71xx: Fix gigabit switch support for Mikrotik RB951G-2HnD

Without this patch, when using rev 3 of the Atheros AR9344 SoC, the
gigabit switch (AR8327) does not work or works very erratically.

This is a re-spin of http://patchwork.ozlabs.org/patch/419857/ with a
different PLL value, according to the feedback from several users
(including myself) as shown here:

  https://openwrt.org/toh/mikrotik/rb2011uias#tracking_reported_experience_with_suggested_patch_for_the_5_gige_ports

Performance is acceptable: testing L3 forwarding without NAT yields a
performance of 370 Mbit/s (iperf3 TCP) and 41 Kpps (iperf3 UDP with 64
bytes payload). Both tests show that 100% of CPU time is spent on softirq.

A similar fix for a different device (RB2011) was added in e457d22261
("Make GBit switch work on RB2011").

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>