board: phycore_imx8mp: Use 2GHz RAM timings for PCL-070 from pcb_rev 1

We need to differ between PCL-070 and PCM-070. PCL-070 supports 2GHz RAM
timings from pcb rev 1 or newer. PCM-070 supports 2GHz RAM timings from
pcb rev 3 or newer.

Signed-off-by: Benjamin Hahn <B.Hahn@phytec.de>