MAINTAINERS: list DW XPCS platform driver and binding under XPCS entry

The Synopsys DW XPCS entry covers pcs-xpcs.c, pcs-xpcs.h and the public
header, but not pcs-xpcs-plat.c or its DT binding. pcs-xpcs-plat.c only
matches the ETHERNET PHY LIBRARY entry today, and the binding is only
matched by the generic DT and networking rules.

Add both files so the entry covers the whole driver.

Signed-off-by: Coia Prant <coiaprant@gmail.com>
Link: https://patch.msgid.link/20261023075214.1793922-6-coiaprant@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>