MAINTAINERS: Add BCM6813 to bcmbca arch entry

Add BCM6813 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

@@ -3792,6 +3792,7 @@ N:	bcm[9]?63148
 N:	bcm[9]?63158
 N:	bcm[9]?63178
 N:	bcm[9]?6756
+N:	bcm[9]?6813
 N:	bcm[9]?6846
 N:	bcm[9]?6855
 N:	bcm[9]?6856