ARM: davinci: make argument to davinci_common_init() as const

Make the function argument of the function davinci_common_init
as const as it's memory contents are only copied during a
memcpy call. So, the fields of the structure to which the argument
soc_info points to never gets modified and therefore the argument can
be made const.
Add const to the prototype too.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>