devicetree.h: add accessor API for nodes and properties

This is joint work with Kumar Gala (see signed-off-by).

Add helper macros which abstract the "true names" of each of the four
types of node identifier we intend to support (e.g. DT_ALIAS(),
DT_INST()).

These can be passed to a new DT_PROP() macro which can be used to read
the value of a devicetree property given a node identifier from one of
these four other macros, and the as-a-c-token name of the property.
Add other accessor macros and tests as well.

Add some convenience APIs for writing device drivers based on instance
numbers as well. Drivers can "#define DT_DRV_COMPAT driver_compatible"
at the top of the file, then utilize these DT_INST_* macros to access
various property defines.

For example, the uart_sifive driver can do:

  #define DT_DRV_COMPAT sifive_uart0

Then use DT_INST macros like:

  .port         = DT_INST_REG_ADDR(0),
  .sys_clk_freq = DT_INST_PROP(0, clock_frequency),

For convenience working with specific hardware, also add:

  <devicetree/gpio.h>
  <devicetree/adc.h>
  <devicetree/spi.h>

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>