t81xx: Migrate TI81XX/TI816X/TI814X symbols to Kconfig

t81xx: Migrate TI81XX/TI816X/TI814X symbols to Kconfig

The symbol CONFIG_TI81XX is used for the parts that are common to the
TI816x and TI814x SoCs and are not part of CONFIG_ARCH_OMAP2PLUS nor
CONFIG_AM33XX.  It however has so few uses that we can just modify the
code to check for both and drop the symbol. The symbols CONFIG_TI816X
and CONFIG_TI814X are for the repective SoCs.

Signed-off-by: Tom Rini <trini@konsulko.com>