MAINTAINERS: Add some unowned files to the SBSA-REF section

These files belong to the sbsa-ref machine and thus should
be listed here.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

MAINTAINERS

@@ -945,6 +945,9 @@ R: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
 L: qemu-arm@nongnu.org
 S: Maintained
 F: hw/arm/sbsa-ref.c
+F: hw/misc/sbsa_ec.c
+F: hw/watchdog/sbsa_gwdt.c
+F: include/hw/watchdog/sbsa_gwdt.h
 F: docs/system/arm/sbsa.rst
 F: tests/avocado/machine_aarch64_sbsaref.py