drivers: uart: esp32: convert to DT_INST defines

Convert driver to fully use DT_INST_ defines.  Currently IRQs are not
generated from DTS on esp32 so we create #defines for what they should
look like.  Convert the IRQ defines to match DT_INST style so if/when
we have that info in DTS it will look the same.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>