softpll-unify: rename timer.c to timer-wrs.c (no effect on rt_cpu.bin)

The commit is part of the effort in unifying softpll with wrpc-sw,
and later remove the duplicated code here in wr-switch-sw.

While we have no gpio.h in wrpc-sw, it's better to clarify this
header is for wr-switch.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>