MAINTAINERS: BCM53573: Add internal Broadcom mailing list

There is a patchwork instance behind bcm-kernel-feedback-list that is
helpful to track submissions, add this list for the Broadcom BCM53573
architecture.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

MAINTAINERS

@@ -3140,6 +3140,7 @@ F:	arch/arm/boot/dts/bcm953012*
 
 BROADCOM BCM53573 ARM ARCHITECTURE
 M:	Rafał Miłecki <rafal@milecki.pl>
+L:	bcm-kernel-feedback-list@broadcom.com
 L:	linux-arm-kernel@lists.infradead.org
 S:	Maintained
 F:	arch/arm/boot/dts/bcm53573*