dt-bindings: mtd: brcm,brcmnand: update interrupts description

1. DMA and EDU are mutually exclusive: don't allow both at the same time
2. Require interrupt-names for 2+ interrupts to avoid /guessing/

Reported-by: Florian Fainelli <f.fainelli@gmail.com>
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/20221104085316.24499-1-zajec5@gmail.com