ARM: dts: koelsch: Correct clock frequency of X2 DU clock input

The X2 crystal oscillator on the Koelsch development board provides a
74.25 MHz clock, not a 148.5 MHz clock.

Fixes: cd21cb46 ("ARM: shmobile: koelsch: Add DU external pixel clocks to DT")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>