ARM: OMAP2+: am33xx: Fix the timer fck clock naming convention

With commit ae6df418
Sub: ARM: OMAP2+: dmtimer: cleanup fclk usage)
The Timer functional clock naming convention has changed from
gptX_fck => timerXfck, and so as the timer init function
in mach-omap2/timer.c.
OMAP4 clocktree also has changed accordingly.

AM33xx Clock Tree has been merged during rc3-4 timeframe,
before above commit got merged, so similar change is required
for AM33xx as well (Change the gptX_fck => timerX_fck).

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>