85xx: get_tbclk() speed up and rounding fix

Speed up get_tbclk() by referencing pre-computed bus clock
frequency value from global data instead of sys_info_t.  Fix
rounding of result to nearest; previously it was rounding
upwards.

Signed-off-by: James Yang <James.Yang@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>