board: verdin-imx8mp: fix lpddr4 refresh timing

Change tRFCmin (tRFCab) from 280 ns to 380 ns to be compliant with
current and futures memories.

Fixes: 2bc2f817 ("board: toradex: add verdin imx8m plus support")

Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>