ARM: dts: Add board-specific compatible string to npcm750-evb devicetree

According to the revised binding, the devicetree needs a board-specific
compatible string.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20210320164023.614059-2-j.neuschaefer@gmx.net


Signed-off-by: Joel Stanley <joel@jms.id.au>