mtd: parsers: add Broadcom's U-Boot parser

Broadcom stores environment variables blocks inside U-Boot partition
itself. This driver finds & registers them.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220711153041.6036-2-zajec5@gmail.com