This PR is preliminary ground work for FMUv5.
PX4 does not use the NuttX adc driver. But used the same format
for the data returned by the nuttx ADC driver.
There was a fixme:in src/platforms/px4_adc.h "this needs to be
a px4_adc_msg_s type" With this PR the need for
src/platforms/px4_adc.h goes away as the driver drv_adc.h now
describes the px4_adc_msg_t.
Daniel Agar