drm/bridge: samsung-dsim: Use HZ_PER_MHZ macro from units.h

Drop the custom MHZ macro and replace it with HZ_PER_MHZ.

Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Link: https://lore.kernel.org/r/20250527-samsung-dsim-v1-2-5be520d84fbb@pengutronix.de


Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>