board: amlogic: odroid: add runtime detection of the N2/N2+/C4/HC4 variants

board: amlogic: odroid: add runtime detection of the N2/N2+/C4/HC4 variants

Use the ADC channel 1 to check the hardware revision of the board and
detect the N2 vs. N2+ and the C4 vs. HC4 variants. Each of them use
different dtb file, so adjust fdtfile environment variable to the
detected variant.

The ADC min/max values for each variant are taken from the vendor code,
adjusted to the 12-bit ADC driver operation mode (vendor code use 10-bit
mode).

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I702a531c24b0fa9bdc1618defa1e0cc2b55c81c5