ARM: dts: mvebu: Update comment for main PLL frequency

The actual frequency was updated in commit ae142bd9

 ("ARM: mvebu:
Fix the main PLL frequency on Armada 375, 38x and 39x SoCs") but the
comment was not updated. Update it now.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>