board: phytec: phycore_imx8mp: Update 2GB RAM Timings

Due to PCB layout constraints in PCB revisions until including 1549.2,
a RAM frequency of 2 GHz can cause rare instabilities. Set the RAM
frequency to 1.5 GHz to achieve a stable system under all conditions.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Yannic Moog <y.moog@phytec.de>
Tested-by: Yannic Moog <y.moog@phytec.de>