ARM: davinci: fix const warnings

After VPIF was converted to enable getting subdevs from DT, the
pdata is no longer const, so remove these to avoid compiler warnings.

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
[nsekhar@ti.com: minor commit message fixup]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>