MAINTAINERS: Add entry for APM X-Gene SoC PMU driver

This patch adds the MAINTAINERS entry for APM X-Gene SoC PMU driver.

Signed-off-by: Tai Nguyen <[e-mail]>

MAINTAINERS

···
 F:	Documentation/devicetree/bindings/net/apm-xgene-enet.txt
 F:	Documentation/devicetree/bindings/net/apm-xgene-mdio.txt
 
+APPLIED MICRO (APM) X-GENE SOC PMU
+M:	Tai Nguyen <[e-mail]>
+S:	Supported
+F:	drivers/perf/xgene_pmu.c
+F:	Documentation/perf/xgene-pmu.txt
+F:	Documentation/devicetree/bindings/perf/apm-xgene-pmu.txt
+
 APTINA CAMERA SENSOR PLL
 M:	Laurent Pinchart <[e-mail]>
 L:	[e-mail]