Merge branch 'bugfix/rmt_mem_struct' into 'master'

Merge branch 'bugfix/rmt_mem_struct' into 'master'

Only fix rmt_mem_t struct definition error.

Modify the struct definition according to technical_reference_manual.
Tested.

```
typedef volatile struct {
    struct {
        union {
            struct {
                uint32_t duration0:    15;
                uint32_t level0:       1;
                uint32_t duration1:    15;
                uint32_t level1:       1;
            };
            uint32_t val;
        } data[64];
    } chan[8];
} rmt_mem_t;
```

See merge request !186