boards/x86/gpmrb: add Gordon Peak MRB board

Add board support (and documentation) for the Intel Gordon Peak
Module Reference Board, a dev board based on the Apollo Lake SoC.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>

soc/x86/apollo_lake/doc/supported_features.rst

@@ -0,0 +1,57 @@
+Supported Features
+==================
+
+In addition to the standard architecture devices (HPET, local and I/O APICs,
+etc.), Zephyr supports the following Apollo Lake-specific SoC devices:
+
+* HSUART
+
+* GPIO
+
+* I2C
+
+HSUART High-Speed Serial Port Support
+-------------------------------------
+
+The Apollo Lake UARTs are NS16550-compatible, with "high-speed" capability.
+
+Baud rates beyond 115.2kbps (up to 3.6864Mbps) are supported, with additional
+configuration. The UARTs are fed a master clock which is fed into a PLL which
+in turn outputs the baud master clock. The PLL is controlled by a per-UART
+32-bit register called ``PRV_CLOCK_PARAMS`` (aka the ``PCP``), the format of
+which is:
+
++--------+---------+--------+--------+
+| [31]   | [30:16] | [15:1] | [0]    |
++========+=========+========+========+
+| enable | ``m``   | ``n``  | toggle |
++--------+---------+--------+--------+
+
+The resulting baud master clock frequency is ``(n/m)`` * master.
+
+Typically, the master clock is 100MHz, and the firmware by default sets
+the ``PCP`` to ``0x3d090240``, i.e., ``n = 288``, ``m =  15625``, which
+results in the de-facto standard 1.8432MHz master clock and a max baud rate
+of 115.2k.  Higher baud rates are enabled by changing the PCP and telling
+Zephyr what the resulting master clock is.
+
+Use devicetree to set the value of the ``PRV_CLOCK_PARAMS`` register in
+the UART block of interest. Typically an overlay ``up_squared.overlay``
+would be present in the application directory, and would look something
+like this:
+
+   .. code-block:: console
+
+    / {
+        soc {
+            uart@0 {
+                pcp = <0x3d090900>;
+                clock-frequency = <7372800>;
+                current-speed = <230400>;
+            };
+        };
+    };
+
+The relevant variables are ``pcp`` (the value to use for ``PRV_CLOCK_PARAMS``),
+and ``clock-frequency`` (the resulting baud master clock). The meaning of
+``current-speed`` is unchanged, and as usual indicates the initial baud rate.