esp32s2: peripherals: store address of timer peripheral in its struct

.. this is helpful if we need to go down to the esp-idf low level
I/O routines

ports/esp32s2/peripherals/timer.c

@@ -68,6 +68,8 @@ void peripherals_timer_init(const timer_config_t *config, timer_index_t *timer)
         }
     }
 
+    timer->hw = (timer->group == 0) ? &TIMERG0 : &TIMERG1;
+
     // initialize timer module
     timer_init(timer->group, timer->idx, config);
     timer_set_counter_value(timer->group, timer->idx, 0);

ports/esp32s2/peripherals/timer.h

@@ -30,6 +30,7 @@
 #include "driver/timer.h"
 
 typedef struct {
+    timg_dev_t *hw;
     timer_idx_t idx;
     timer_group_t group;
 } timer_index_t;