arm: am33xx: Kconfig: Add secure device definitions

Adds a new Kconfig file for AM33xx class devices. We
need a common place to define CONFIG parameters
for these SOCs, especially for adding support
for secure devices.

a) Adds a definition for ISW_ENTRY_ADDR. This is the
address to which the ROM branches when the SOC
ROM hands off execution to the boot loader.
CONFIG_SYS_TEXT_BASE and CONFIG_SPL_TEXT_BASE are set
to this value for AM43xx devices.

b) Adds CONFIG_PUB_ROM_DATA_SIZE which is used to
calculate CONFIG_SPL_MAX_SIZE. This value indicates the
amount of memory needed by the ROM to store data during
the boot process.

Currently, these CONFIG options are used only by AM43xx,
but in future other AM33xx class SOCs will also use them.

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

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