Revert "dt-bindings: marvell,mmp2: Add clock id for the SP clock"

It seems that the kernel has no business managing this clock: once the SP
clock is disabled, it's not sufficient to just enable it in order to bring
the SP core back up.

Pretty sure nothing ever used this and it's safe to remove.

This reverts commit e8a2c779.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>