This patch does following:
1. Adds a new cros-ec-uart driver. This driver can send EC requests on
   UART and process response packets received on UART transport.
2. Once probed, this driver will initialize the serdev device based on
   the underlying information in the ACPI resource. After serdev device
   properties are set, this driver will register itself cros-ec.
3. High level driver can use this implementation to talk to ChromeOS
   Embedded Controller device in case it supports UART as transport.
4. When cros-ec driver initiates a request packet, outgoing message is
   processed in buffer and sent via serdev. Once bytes are sent, driver
   enables a wait_queue.
5. Since ChromeOS EC device sends response asynchronously, AP's TTY
   driver accumulates response bytes and calls the registered callback.
   TTY driver can send multiple callback for bytes ranging from 1 to MAX
   bytes supported by EC device.
6. Driver waits for EC_MSG_DEADLINE_MS to collect and process received
   bytes. It wakes wait_queue if expected bytes are received or else
   wait_queue timeout. Based on the error condition, driver returns
   data_len or error to cros_ec.
Signed-off-by: Bhanu Prakash Maiya <bhanumaiya@chromium.org>
Co-developed-by: Mark Hasemeyer <markhas@chromium.org>
Signed-off-by: Mark Hasemeyer <markhas@chromium.org>
Reviewed-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20221227123212.v13.1.If7926fcbad397bc6990dd725690229bed403948c@changeid