ARM: OMAP3: cm-t3517: use GPTIMER for system clock

cm-t3517 starting from revision 1.2 does not have the 32K oscilator
wired to the AM3517 SoC.
Therefore switch to use the GPTIMER for system clock.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>