ARM: rmobile: Enable KSZ90X1 PHY on Salvator-X

Commit da3b9e7f (Move PHY_MICREL and PHY_MICREL_KSZ90X1 to Kconfig)
broke ethernet support on R8A779x Salvator-X boards by disabling the
KSZ90x1 PHY in Kconfig. Reenable it.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Alexandru Gagniuc <alex.g@adaptrum.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>