omap2+: Rename timer-gp.c into timer.c to combine timer init functions

We can keep everything sys_timer and gptimer.c related code in
timer.c as the code will be very minimal.

Later on we can also remove timer-mpu.c, as it can be called from
omap4_timer_init function.

This allows us to get rid of confusing existing files. We currently
have timer-gp.c, timer-mpu.c, and patches have been posted to add
dmtimer.c. There's no need to have these multiple files, we can
put everything into timer.c.

Signed-off-by: Tony Lindgren <tony@atomide.com>