MAINTAINERS: Add Jon Mason to BCM5301X maintainers

Adding myself as a maintainer for Broadcom Northstar SoCs.  Also, adding
a regex entry to discover the SVK DTs already present.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>