arch: powerpc: update the eLBC IP input clock

eLBC IP clock is always a constant divisor of platform clock
pre-defined per SoC. Clock ratio register (LCRR) used in
current implementation governs eLBC IP output cloc.

Update sys_info->freq_localbus to represent eLBC input clock with
value constant divisor of platform clock.

Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>