imx-common: timer: clean up code

We can reuse common functions in lib/time.c, but not reimplement
functions in imx-common/time.c.
Only keep timer_init ,get_tbclk and implement timer_read_counter in
imx-common/time.c.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>