stm32/modmachine: Only enable machine.I2C if hardware I2C is enabled.

Prior to this change, if a board did not define any hardware I2C pins, via
MICROPY_HW_I2Cx_SCL, then machine.I2C would alias to machine.SoftI2C.

That doesn't really make sense, and SoftI2C should always be used if there
is no hardware implementation.  So this commit makes it so that machine.I2C
is only available if at least one set of I2C hardware pins are defined via
the MICROPY_HW_I2Cx_SCL/SDA macros.

For all boards that define at least one set of I2C hardware pins (which is
most of them) this commit is a no-op.  The only boards that change are:
LEGO_HUB_NO6, LEGO_HUB_NO7, STM32H7B3I_DK.
Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/mpconfigport.h

@@ -117,7 +117,7 @@
 #endif
 #define MICROPY_PY_MACHINE_PULSE    (1)
 #define MICROPY_PY_MACHINE_PIN_MAKE_NEW mp_pin_make_new
-#define MICROPY_PY_MACHINE_I2C      (1)
+#define MICROPY_PY_MACHINE_I2C      (MICROPY_HW_ENABLE_HW_I2C)
 #define MICROPY_PY_MACHINE_SOFTI2C  (1)
 #define MICROPY_PY_MACHINE_I2S_INCLUDEFILE "ports/stm32/machine_i2s.c"
 #define MICROPY_PY_MACHINE_I2S_CONSTANT_RX (I2S_MODE_MASTER_RX)