fix ADC instruction when used with undocumented parameter

binutil-esp32ulp accepts an undocumented 4th parameter for the ADC
instruction, even though this parameter is unused as per binutils'
current code.

This commit contributes to being able to eventually assemble the
esp32ulp_all.s test from binutils-esp32ulp. It addresses this line:
https://github.com/espressif/binutils-esp32ulp/blob/249ec34cc2c9574a86f3f86bbb175a863f988bcf/gas/testsuite/gas/esp32ulp/esp32/esp32ulp_all.s#L174

Author: wnienhaus

esp32_ulp/opcodes.py

@@ -449,7 +449,7 @@ def i_tsens(reg_dest, delay):
     return _tsens.all
 
 
-def i_adc(reg_dest, adc_idx, mux):
+def i_adc(reg_dest, adc_idx, mux, _not_used=None):
     _adc.dreg = get_reg(reg_dest)
     _adc.mux = get_imm(mux)
     _adc.sar_sel = get_imm(adc_idx)

tests/compat/fixes.S

@@ -28,4 +28,9 @@ entry:
   # interpret ; as statement separator - this results in 2 NOP machine instructions
   nop; nop;
 
+  # adc supports an undocumented 4th argument, which should be entirely ignored
+  # binutils-esp32ulp also ignores this argument, if present, see:
+  # https://github.com/espressif/binutils-esp32ulp/blob/249ec34cc2c9574a86f3f86bbb175a863f988bcf/gas/config/esp32ulp-parse.y#L810
+  adc r1, 0, 1, 100
+
   halt