arm: tcc8k: Fix bus clock calculation

There are two dividers used to derive bus clock from system clock:
system clock is divided by SCKDIV+1, then by BCKDIV+1. SCKDIV divider
has been ignored up to now, which is no problem as long as it is 0.

Take SCKDIV into account for bus clock calculation.

Signed-off-by: Oskar Schirmer <oskar@linutronix.de>
Cc: bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>