apm821xx: MR24: fix ethernet phy detection on the MR24

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This patch fixes a problem where the AR8035 PHY can't be
detected on the Cisco Meraki MR24, when the ethernet cable
is not connected during boot.

Russell Senior reported:
|This appears to be a problem during probing of the AR8035
|phy chip. When ethernet has no link, the phy detection fails,
|and eth0 is not created. Plugging ethernet later has no effect,
|because there is no interface as far as the kernel is
|concerned. The relevant part of the boot log looks like this:
|
|[    0.876611] /plb/opb/emac-rgmii@ef601500: input 0 in RGMII mode
|[    0.882532] /plb/opb/ethernet@ef600c00: reset timeout
|[    0.888546] /plb/opb/ethernet@ef600c00: can't find PHY!
(<https://bugs.lede-project.org/index.php?do=details&task_id=687>)

Fixes FS#687
Cc: Chris Blake <chrisrblake93@gmail.com>
Reported-by: Russell Senior <russell@personaltelco.net>
Fixes: 23fbb5a87c56e98 ("emac: Fix EMAC soft reset on 460EX/GT")
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>