arm: davinci: remove leftover code for dm* SoCs

The support for DaVinci DM* SoCs has been dropped a while ago. There's
still a lot of leftover code in mach-davinci though. Entirely remove
certain files and modify the common code to no longer reference
unsupported chips.

Note: all DaVinci platforms supported in u-boot now define SOC_DA8XX
but not all define SOC_DA850 (e.g. omapl138). We can safely remove
all ifdefs for the former, but let's leave the ones for the latter.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>