brcmfmac: Add support for getting nvram contents from EFI variables

Various X86 laptops with a SDIO attached brcmfmac wifi chip, store the
nvram contents in a special EFI variable. This commit adds support for
getting nvram directly from this EFI variable, without the user needing
to manually copy it.

This makes Wifi / Bluetooth work out of the box on these devices instead of
requiring manual setup.

This has been tested on the following models: Acer Iconia Tab8 w1-810,
Acer One 10, Asus T100CHI, Asus T100HA, Asus T100TA, Asus T200TA and a
Lenovo Mixx 2 8.

Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>