[media] s5p-csis: Change regulator supply names

Rename the regulator supply names to more meaningful ones.
It's a prerequisite for adding device tree support.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>