kconfig: define ESP32_PHY_MAX_TX_POWER if PHY_ENABLED

kconfig: define ESP32_PHY_MAX_TX_POWER if PHY_ENABLED

ESP32_PHY_MAX_TX_POWER option is only meaningful for WiFi, so previous
change made it depend on WIFI_ENABLED. However if WiFi is not enabled,
but BT is, this option becomes undefined which breaks phy_init_data
generation.

This change turns ESP32_PHY_MAX_TX_POWER into a hidden parameter, which
depends on PHY_ENABLED. New user-visible parameter,
ESP32_PHY_MAX_WIFI_TX_POWER is introduced which depends on WIFI_ENABLED
and is used as default value for ESP32_PHY_MAX_TX_POWER if WIFI_ENABLED
is set. Otherwise ESP32_PHY_MAX_WIFI_TX_POWER is set to 20.