drivers: wifi: esp_at: rename driver from esp

Recently WiFi ESP32 driver (utilizing WiFi radio in ESP32 SoC) was
introduced into drivers/wifi/esp32/ and it already caused confusion as
there was existing drivers/wifi/esp/ directory for ESP-AT
driver (utilizing external WiFi chip, by communicating using AT commands
from any serial capable platform). So question has arisen whether it is
good to merge both, while they are totally different drivers.

Rename ESP-AT driver to be placed in drivers/wifi/esp_at/, so that it is
easier to figure out difference between "esp32" and "esp_at" just by
looking at driver name. Rename also DT compatible and all Kconfig
options for the same reason.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>