arm64: dts: marvell: armada-37xx: Add xtal clock to comphy node

Kernel driver phy-mvebu-a3700-comphy.c needs to know the rate of the
reference xtal clock. So add missing xtal clock source into comphy device
tree node. If the property is not present, the driver defaults to 25 MHz
xtal rate (which, as far as we know, is used by all the existing boards).

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>