extmod/machine_spi: Make SPI protocol structure public.

So it can be referenced directly without the need for the uPy object.

extmod/machine_spi.c

@@ -321,7 +321,7 @@ STATIC void mp_machine_soft_spi_init(mp_obj_base_t *self_in, size_t n_args, cons
     mp_hal_pin_input(self->miso);
 }
 
-STATIC const mp_machine_spi_p_t mp_machine_soft_spi_p = {
+const mp_machine_spi_p_t mp_machine_soft_spi_p = {
     .init = mp_machine_soft_spi_init,
     .deinit = NULL,
     .transfer = mp_machine_soft_spi_transfer,

extmod/machine_spi.h

@@ -46,6 +46,7 @@ typedef struct _mp_machine_soft_spi_obj_t {
     mp_hal_pin_obj_t miso;
 } mp_machine_soft_spi_obj_t;
 
+extern const mp_machine_spi_p_t mp_machine_soft_spi_p;
 extern const mp_obj_type_t mp_machine_soft_spi_type;
 extern const mp_obj_dict_t mp_machine_spi_locals_dict;