iot2050: Add support for configuring M.2 connector

The M.2 slots of the related IOT2050 variant need to be configured
according to the plugged cards. This tries to detect the card using the
M.2 configuration pins of the B-key slot. If that fails, a U-Boot
environment variable can be set to configure manually. This variable is
write-permitted also in secure boot mode as it is not able to undermine
the integrity of the booted system.

The configuration is then applied to mux the serdes and to fix up the
device tree passed to or loaded by the bootloader. The fix-ups are
coming from device tree overlays that are embedded into the firmware
image and there also integrity protected. The OS remains free to load
a device tree to which they do not apply: U-Boot will not fail to boot
in that case.

Based on original patch by Chao Zeng.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>