Kconfig: Separate AM33XX SOC config from target board config

The config option AM33XX is used in several boards and should be
defined as a stand-alone option for this SOC. We break this out
from target boards that use this SoC and common headers then enable
AM33XX on in all the boards that used these targets to eliminate any
functional change with this patch.

This is similar to what has already been done in
9de852642cae ("arm: Kconfig: Add support for AM43xx SoC specific Kconfig")
and is done for the same reasons.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>