ARM: OMAP2+: free use_gptimer_clksrc variable after boot

The variable use_gptimer_clksrc is only used by two __init functions,
So we can freely free it after boot.

Signed-off-by: Oussama Ghorbel <ghorbel@pivasoftware.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>