x86: coral: Show memory config and SKU ID on startup

Provide the model information through sysinfo so that it shows up on
boot. For memconfig 4 pins are provided, for 16 combinations. For SKU
ID there are two options:

- two pins provided in a ternary arrangement, for 9 combinations.
- reading from the EC

Add a binding doc and drop the unused #defines as well.

Example:

U-Boot 2021.01-rc5

CPU: Intel(R) Celeron(R) CPU N3450 @ 1.10GHz
DRAM: 3.9 GiB
MMC: sdmmc@1b,0: 1, emmc@1c,0: 2
Video: 1024x768x32 @ b0000000
Model: Google Coral (memconfig 5, SKU 3)

This depends on the GPIO series:

http://patchwork.ozlabs.org/project/uboot/list/?series=228126

Signed-off-by: Simon Glass <sjg@chromium.org>

Acked-by: Bin Meng <bmeng.cn@gmail.com>