drivers: serial: esp32: rename fake DT define

Rename DT_INST_{0,1,2}_ESPRESSIF_ESP32_UART_IRQ_0 defines to something
non-DT prefixed.  This way we know which defines are one's we generate
and which ones are driver created.  It should be easy enough to replace
these INST_{0,1,2}_ESPRESSIF_ESP32_UART_IRQ_0 define with DTS generated
one macros once esp32 has interrupt controller support in DTS.

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