Enables a usable adc driver instance based on the board design and
configures the pinmux to enable one adc channel on the SoC's PTB2 pin,
which is routed to the Arduino header A2 pin. Updates the board
documentation to reflect that the board now supports an adc driver.
Jira: ZEP-1396
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>
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