ARM: rmobile: Zap CONFIG_SYS_CLK_FREQ where applicable

The CONFIG_SYS_CLK_FREQ is not used on some of the Gen3 boards,
remove it. Moreover, on Ebisu this actually didn't match the
comment in the config file at all, but since it was not used,
there was no real problem.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>