[D18][DT] usdhc2: increase drive-strength for uSD

The signal on some uSD seems to undershoot too much at 50MHz with the original drive-strength X1: increase it to X4, as X2 is also not sufficient.

As 100MHz is set to X2, increase it too to X4 to be safe.

Drive Strength as described in IMX8MPRM:

0x0 -> 00* == DSE_X1 — X1
0x4 -> 10* == DSE_X2 — X2
0x2 -> 01* == DSE_X4 — X4
0x6 -> 11* == DSE_X6 — X6