0c18184d ("platform/x86: apple-gmux: support MMIO gmux on T2 Macs")
brought support for T2 Macs in apple-gmux. But in order to use dual GPU,
the integrated GPU has to be enabled. On such dual GPU EFI Macs, the EFI
stub needs to report that it is booting macOS in order to prevent the
firmware from disabling the iGPU.

This patch is also applicable for some non T2 Intel Macs.

Based on this patch for GRUB by Andreas Heider <andreas@heider.io>:
https://lists.gnu.org/archive/html/grub-devel/2013-12/msg00442.html

Credits also goto Kerem Karabay <kekrby@gmail.com> for helping porting
the patch to the Linux kernel.

Cc: Orlando Chamberlain <orlandoch.dev@gmail.com>
Signed-off-by: Aditya Garg <gargaditya08@live.com>
[ardb: limit scope using list of DMI matches provided by Lukas and Orlando]
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Tested-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>