ARM: OMAP2+: Fix booting if no timer parent clock is available

When bringing up a new SoC we needlessly prevent booting at timer
init if timer clock_set_parent fails. This can fail if the system
is booting on bootloader configured PLL values until the clock
framework driver for the PLL is implemented.

Let's just WARN instead, this will provide helpful information
for anybody bringing up a new SoC what needs to be fixed.

This allows to boot dm814x that's still missing the PLL driver.

Signed-off-by: Tony Lindgren <tony@atomide.com>