ARM: shmobile: r7s72100: fix bus clock calculation

The picture in the datasheet is a little misleading, yet the divider of
the bus_clk is 1/3 and not 2/3.

Signed-off-by: Wolfram Sang <wsa@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>