MAINTAINERS: add entry for Rockchip XPCS driver

Add a MAINTAINERS entry for the Rockchip RK3568 XPCS platform driver
and its device tree binding.

Include the relevant mailing lists (netdev and linux-rockchip) so that
future patches are properly distributed.

Signed-off-by: Coia Prant <coiaprant@gmail.com>
Link: https://patch.msgid.link/20261017054406.1828061-7-coiaprant@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>