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