arm: Kconfig: Add support for AM43xx SoC specific Kconfig

Adding support for AM43xx secure devices require the addition
of some SOC specific config options like the amount of memory
used by public ROM and the address of the entry point of u-boot
or SPL, as seen by the ROM code, for the image to be built
correctly.

This mandates the addition of am AM43xx CONFIG option and the
ARM Kconfig file has been modified to source this SOC Kconfig
file. Moving the TARGET_AM43XX_EVM config option to the SOC
KConfig and out of the arch/arm/Kconfig.

Updating defconfigs to add the CONFIG_AM43XX=y statement and
removing the #define CONFIG_AM43XX from the header file.

Signed-off-by: Madan Srinivas <madans@ti.com>
Signed-off-by: Daniel Allred <d-allred@ti.com>

Tested-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>