MAINTAINERS: adjust file entry in DRM ACCEL DRIVER FOR ROCKCHIP NPU

Commit a7352c84 ("dt-bindings: npu: rockchip,rknn: Add bindings") adds
the device-tree binding rockchip,rk3588-rknn-core.yaml, whereas the commit
ed98261b ("accel/rocket: Add a new driver for Rockchip's NPU") adds the
section DRM ACCEL DRIVER FOR ROCKCHIP NPU in MAINTAINERS with a file entry
referring to rockchip,rknn-core.yaml. Note that the file entry is missing
the part rk3588, compared to the added file above, which it intends to
refer to.

Adjust the file entry to the intended file name.

Fixes: ed98261b ("accel/rocket: Add a new driver for Rockchip's NPU")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Tomeu Vizoso <tomeu@tomeuvizoso.net>
Link: https://lore.kernel.org/r/20250826063248.32153-1-lukas.bulwahn@redhat.com