arm: rmobile: kzm9g: Adjust ETM trace clock

Set ETM TRCLK down to 78MHz to get clear wave form.
This patch makes difference only when you use ETM trace connecting JTAG debugger.

Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
Acked-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>