Bluetooth: controller: enable dynamic TXP over LL_SPLIT arch (#17731)

This commit targets solving issue #17731 over the LL_SW_SPLIT
arch of the BLE stack in Zephyr. This functionality is exposed
to the user as HCI Zephyr Command extensions

- BT_HCI_OP_VS_WRITE_TX_POWER_LEVEL
- BT_HCI_OP_VS_READ_TX_POWER_LEVEL

which enable Tx power read/write operations within BLE radio events
on a per role/connection basis.

The functionality is enabled upon the Kconfig advanced configuration
triggered by

- BT_CTLR_TX_PWR_DYNAMIC_CONTROL

depending on the enablement of Zephyr HCI vendor-specific command
extensions.

Necessary low-level radio HAL functionality and power definitions
are also supplied to address the high-level functionality of
controlling the Tx power.

Signed-off-by: Andrei Stoica <stoica.razvan.andrei@gmail.com>

subsys/bluetooth/controller/ll_sw/lll_conn.h

@@ -112,10 +112,15 @@ struct lll_conn {
 #if defined(CONFIG_BT_CTLR_CONN_META)
 	struct lll_conn_meta conn_meta;
 #endif /* CONFIG_BT_CTLR_CONN_META */
+
+#if defined(CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL)
+	s8_t tx_pwr_lvl;
+#endif
 };
 
 int lll_conn_init(void);
 int lll_conn_reset(void);
+
 u8_t lll_conn_sca_local_get(void);
 u32_t lll_conn_ppm_local_get(void);
 u32_t lll_conn_ppm_get(u8_t sca);