dt-bindings: net: bluetooth: nxp: Add missing type for "fw-init-baudrate"

"fw-init-baudrate" is missing a type, add it. While we're here, define the
default value with a schema rather than freeform text.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230613200929.2822137-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>