ARM: dts: Provide WLAN firmware names for Zero 2 W

BCM43430/2 may be BCM43430B0 or BCM43436P, and BCM43430/1 can be either
BCM43430A1 or BCM43436S, the former being upstream names and the
latter downstream names for differently-sourced sister parts.

Make the choice of firmwares board-specific (without making the actual
firmwares board-specific) by placing the alternative firmware names for
each part in the DT node.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>