wifi: brcmfmac: of: Fetch Apple properties

On Apple ARM64 platforms, firmware selection requires two properties
that come from system firmware: the module-instance (aka "island", a
codename representing a given hardware platform) and the antenna-sku.
We map Apple's module codenames to board_types in the form
"apple,<module-instance>".

The mapped board_type is added to the DTS file in that form, while the
antenna-sku is forwarded by our bootloader from the Apple Device Tree
into the FDT. Grab them from the DT so firmware selection can use
them.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/E1oZDnn-0077aS-NA@rmk-PC.armlinux.org.uk