Add HL78XX Modem Driver Implementation Using Modem Chat Framework

This pull request introduces the initial draft implementation of the HL78XX modem driver, built on the `modem_chat` framework. The current version establishes the core structure and essential functionality required to support the HL78XX modem series.

The primary objective at this stage is to review and align on the overall design and integration approach. Following this, development will continue with additional features such as:

* Power saving mode support
* User pipe connection handling
* Other modem-specific enhancements
* Verified compatibility and correct operation with the following sample applications:

  * `aws_mqtt`
  * `dns_resolve`
  * `big_http_download`
  * `lwm2m`

Feedback on both structure and test approach is welcome.

Author: zafersn

drivers/modem/CMakeLists.txt

@@ -35,5 +35,7 @@ if (CONFIG_MODEM_SIM7080)
 	zephyr_library_sources(simcom-sim7080.c)
 endif()
 
+add_subdirectory_ifdef(CONFIG_MODEM_HL78XX hl78xx)
+
 zephyr_library_sources_ifdef(CONFIG_MODEM_CELLULAR modem_cellular.c)
 zephyr_library_sources_ifdef(CONFIG_MODEM_AT_SHELL modem_at_shell.c)

drivers/modem/Kconfig

@@ -194,6 +194,7 @@ source "drivers/modem/Kconfig.cellular"
 source "drivers/modem/Kconfig.at_shell"
 
 source "drivers/modem/Kconfig.hl7800"
+source "drivers/modem/hl78xx/Kconfig.hl78xx"
 source "drivers/modem/Kconfig.simcom-sim7080"
 
 endif # MODEM

drivers/modem/hl78xx/CMakeLists.txt

@@ -0,0 +1,16 @@
+#
+#  Copyright (c) 2018 Analog Devices Inc.
+#
+#  SPDX-License-Identifier: Apache-2.0
+#
+zephyr_library()
+
+zephyr_library_sources(
+    hl78xx.c
+    hl78xx_sockets.c
+    hl78xx_utility.c
+
+)
+
+zephyr_library_include_directories(${ZEPHYR_BASE}/subsys/net/ip)
+zephyr_library_include_directories(${ZEPHYR_BASE}/subsys/net/lib/sockets)