MAINTAINERS: add bgmac section entry

This driver exists for years but was missing its MAINTAINERS entry.

Signed-off-by: Rafał Miłecki <[e-mail]>
Acked-by: Florian Fainelli <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

MAINTAINERS

···
 N:	hr2
 N:	stingray
 
+BROADCOM IPROC GBIT ETHERNET DRIVER
+M:	Rafał Miłecki <[e-mail]>
+M:	[e-mail]
+L:	[e-mail]
+S:	Maintained
+F:	Documentation/devicetree/bindings/net/brcm,amac.txt
+F:	drivers/net/ethernet/broadcom/bgmac*
+F:	drivers/net/ethernet/broadcom/unimac.h
+
 BROADCOM KONA GPIO DRIVER
 M:	Ray Jui <[e-mail]>
 L:	[e-mail]