MAINTAINERS: Add BCM6856 to bcmbca arch entry

Add BCM6856 related files to BCMBCA ARCH maintainer list entry

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

MAINTAINERS

@@ -3793,6 +3793,7 @@ N:	bcm[9]?63178
 N:	bcm[9]?6756
 N:	bcm[9]?6846
 N:	bcm[9]?6855
+N:	bcm[9]?6856
 N:	bcm[9]?6858
 N:	bcm[9]?6878