ARM: davinci: Delete DM646x board files

The interest of maintaining the DaVinci DM644x board files seems
very low. Patches to convert the EVM board to use GPIO descriptors
has not been reviewed, tested or merged for several merge
windows in a row, see link below.

When I look in the logs for the board files I see nothing but
generic kernel maintenance and no testing on real hardware for
years.

I conclude the DM646x board files are unused and can be deleted.

Cc: Kevin Hilman <khilman@baylibre.com>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Matt Porter <mporter@konsulko.com>
Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/linux-arm-kernel/20220507124536.171930-2-linus.walleij@linaro.org/


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>