ARM: OMAP: dmtimer: check for fixed timers during config

The omap_dm_timer_set_source() function provides a means for client
users to configure the mux parent for a GPTimer's functional clock.
However, not all timers are configurable (Eg: Timer12 on DRA7 is fed
by an internal 32k oscillator clock, and does not have configurable
parent clocks). So, check for such cases and proceed with out throwing
an error.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>