ARM: OMAP2+: Kconfig: convert SOC_OMAPTI81XX to SOC_TI81XX

No need to have an OMAP prefix on these SoCs that are in the family
but arent' really called OMAP.

Simple rename: CONFIG_SOC_OMAPTI81XX --> CONFIG_SOC_TI81XX

No functional change.

Signed-off-by: Kevin Hilman <khilman@ti.com>
Acked-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>