dt-bindings: mtd: brcm,brcmnand: Drop "brcm,brcmnand" compatible for iProc

Some users of "brcm,nand-iproc" include "brcm,brcmnand" and some don't.
The "brcm,brcmnand" compatible is not useful for iProc systems as
there's a separate driver for iProc. So drop it as a fallback.

Reviewed-by: William Zhang <william.zhang@broadcom.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>