dt-bindings: mtd: brcmnand: Add brcmnand, brcmnand-v7.3 support

Added brcm,brcmnand-v7.3 as possible compatible string to support
brcmnand controller v7.3.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>