MAINTAINERS: Add entry for X-Powers AXP family PMIC drivers

Add an entry for X-Powers AXP family PMIC drivers and list myself
as maintainer.

Signed-off-by: Chen-Yu Tsai <[e-mail]>
Acked-by: Maxime Ripard <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

MAINTAINERS

···
 F:	kernel/workqueue.c
 F:	Documentation/workqueue.txt
 
+X-POWERS MULTIFUNCTION PMIC DEVICE DRIVERS
+M:	Chen-Yu Tsai <[e-mail]>
+L:	[e-mail]
+S:	Maintained
+N:	axp[128]
+
 X.25 NETWORK LAYER
 M:	Andrew Hendry <[e-mail]>
 L:	[e-mail]