include: env: ti: Add a generic default_findfdt.env

include: env: ti: Add a generic default_findfdt.env

ti_mmc bootmethod uses a findfdt routine that is expected to be
implemented by all platforms.

Define a default findfdt based on configured DEFAULT_DEVICE_TREE option
for u-boot. This saves duplication across multiple boards and handles
architecture folder location changes centrally.

TI ARMV7 platforms will need to override default_device_tree_subarch
in the env file to point to the appropriate platform. Note: default
"omap" is used to cater to "most common" default.

Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>