ARM: rmobile: Reset ethernet PHY

Toggle the PHY reset GPIO to bring the ethernet PHY out of reset properly.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
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NOTE: This should be moved to the SH ethernet driver, but it's quite
      late in the cycle, so this is something to be done in 2018.05.