cmd: adc: Add support for storing ADC result in env variable

Add the ability to save ADC conversion result in an environment
variable. This is useful for further arbitrary processing by the
U-Boot scripts.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>