media: ipu3.rst: Improve header formatting on tables

Use the header-rows option of the flat-table directive in order to have
the first row displayed as a header. Also capitalize these headers.
These changes make the tables easier to read.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>