board: beagle: beagleplay: Set fdtfile from C code instead of findfdt script

Stop using the findfdt script and switch to setting the fdtfile from C
code.

Reviewed-by: Jonathan Humphreys <j-humphreys@ti.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>