ARM: dts: vf610-zii-dev-rev-b: Fix MDIO over clocking

The ZII devel B board has two generations of Marvell Switches.  The
mv88e6352 supports an MDIO clock of 12MHz. However the older 88e6185
does not like 12MHz, and often fails to probe.

Reduce the clock speed to 5MHz, which seems to work reliably.

Cc: Chris Healy <cphealy@gmail.com>
Fixes: b9553876 ("ARM: dts: ZII: update MDIO speed and preamble")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>