MAINTAINERS: Add Frank Li as PCI endpoint reviewer

I have volunteered to review PCI endpoint-related changes.  Add myself as a
reviewer to be notified when related patches are posted.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Link: https://patch.msgid.link/20260611210007.529205-1-Frank.Li@oss.nxp.com