mtd: bcm47xxpart: detect block aligned Squashfs partition

Most of the bcm47xx devices use TRX format for storing kernel and some
partition like Squashfs or JFFS2. This is pretty flexible solution, CFE
(the bootloader) just writes (and later boots) TRX at some hardcoded
place and paritions can vary in the size.

However some devices don't use TRX format. Very recently we have
discovered ZTE H218N that has kernel and rootfs partitions at some
"random" places.

This patch allows Linux find a rootfs partition after installing custom
image with a CFE bootloader.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>