dt-bindings: mtd: partitions: add binding for U-Boot bootloader

Right now there is no (known) real reason for a custom binding for
standard U-Boot partitions. Broadcom's U-Boot however requires extra
handling - looking for environment variables subblocks. This commit adds
Broadcom specific binding.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220711153041.6036-1-zajec5@gmail.com