extmod/machine_i2c: Make SoftI2C configurable via macro option.

The zephyr port doesn't support SoftI2C so it's not enabled, and the legacy
I2C constructor check can be removed.

Signed-off-by: Damien George <damien@micropython.org>
diff --git a/ports/esp32/mpconfigport.h b/ports/esp32/mpconfigport.h
index 057251f..b9e1337 100644
--- a/ports/esp32/mpconfigport.h
+++ b/ports/esp32/mpconfigport.h
@@ -159,6 +159,7 @@
 #define MICROPY_PY_MACHINE_BITSTREAM        (1)
 #define MICROPY_PY_MACHINE_PULSE            (1)
 #define MICROPY_PY_MACHINE_I2C              (1)
+#define MICROPY_PY_MACHINE_SOFTI2C          (1)
 #define MICROPY_PY_MACHINE_SPI              (1)
 #define MICROPY_PY_MACHINE_SPI_MSB          (0)
 #define MICROPY_PY_MACHINE_SPI_LSB          (1)