ARM: OMAP2+: Drop useless gptimer option for omap4

We have local timers on Cortex-A9, so using the gptimer option makes no
sense. Let's just drop it for omap4 to simplify the timer options a bit.

If this is really needed, it can be still done by specifying dts properties
in the board specific file for assigned-clocks and assigned-clock-parents.

This gets us a bit closer to start dropping legacy platform data for
gptimers except for timer1 that is used for system clockevent.

Cc: Keerthy <j-keerthy@ti.com>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>