powerpc/85xx: Change timebase divisor to be defined per processor

Introduce new CONFIG_SYS_FSL_TBCLK_DIV on 85xx platforms because
different SoCs have different divisor amounts.  All the PQ3 parts are
/8, the P4080/P4080 is /16, and P2040/P3041/P5020 are /32.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>