am335x_evm: Add support to boot from NOR.

NOR requires that s_init be within the first 4KiB of the image so that
we can perform the rest of the required pinmuxing to talk with the rest
of NOR that we are found on.  When NOR_BOOT is set we save our
environment in NOR at 512KiB and a redundant copy at 768KiB.  We avoid
using SPL for this case and u-boot.bin is written directly to the start
of NOR.

We enclose the DMM-related parts of arch/arm/cpu/armv7/am33xx/emif4.c
with TI81xx checks as at this time U-Boot does not discard unused
sections in the main build and this code relies on functions specific to
(and only provided in) ti81xx-related code.

Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Steve Kipisz <s-kipisz2@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>