MAINTAINERS: Update BCM281XX/BCM11XXX/BCM216XX entry

There are many clock, pinctl, and misc others that should be covered
under the BCM281XX/BCM11XXX/BCM216XX ARM listing.  Change the entry to
use regex's that should cover all the files.

Also, remove the bcm_defconfig entry (as the file is being removed), and
arch/arm64/boot/dts/broadcom reference (as that is not accurate for this
group of maintainers and all the device trees under it should now be
covered by other maintainer entries).

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>