rmobile: Fix build timer driver with BUILD_DIR

Rmobile common timer driver  diverts the same driver as SH architecture.
When it builds at the same place with source, it is no problem, but when
it builds out of source, it cannot build.
This patch revises this problem.

Reported-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>