media: docs: vidioc-g-ext-ctrls.rst: Document p_s32 and p_s64 fields

p_s32 and p_s64 were added to the v4l2_ext_control struct, so these
should have documentation as well.

Signed-off-by: Daniel Lundberg Pedersen <dlp@qtec.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>