Convert the mtd-physmap binding to DT schema format. The arm-versatile,
cypress,hyperflash and intel,ixp4xx-flash are all just an additional
compatible string, so they are all merged into the main schema.

There doesn't appear to be any users nor support for 'vendor-id' and
'device-id', so these have been dropped.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20210607193500.3085920-1-robh@kernel.org