am335x_evm: Add basic README

Add a README for the family of boards the am335x_evm covers, and include
instructions on preparing and using falcon mode, for various media.

Signed-off-by: Tom Rini <trini@ti.com>
Reviewed-by: Peter Korsgaard <jacmet@sunsite.dk>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>