arm: am33xx: Avoid writing into reserved DPLL divider

DPLL DRR doesn't have an M4 divider. But the clock driver is trying
to configure M4 divider as 4(writing into a reserved register).
Fixing it by making M4 divider as -1.

Reported-by: Steve Kipisz <s-kipisz2@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>