ARM: OMAP: dmtimer: use devm_ API and do some cleanup in probe()

Replace the regular kzalloc and ioremap with the devm_ equivalent
to simplify error handling. We don't need kree() any more in
omap_dm_timer_remove().

Also added *dev* pointer to reference pdev->dev which makes the
usage shorter in code.

Cc: Cousson, Benoit <b-cousson@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>