dt-bindings: marvell,mmp2: Add clock id for the fifth SD HCI on MMP3

There's one extra SDHCI on MMP3, used by the internal SD card on OLPC
XO-4. Add a clock for it.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lkml.kernel.org/r/20200309194254.29009-16-lkundrak@v3.sk
Signed-off-by: Stephen Boyd <sboyd@kernel.org>