acpigen_ps2_keybd: Add Fn key to linux,keymap

Some devices may generate scancodes for the Fn key if they have one.
If they do, we should add them to the linux,keymap binding.

BUG=b:333096023
TEST=Flash DUT that emits a scancode for the Fn key, verify that it is
mapped to KEY_FN in the Linux kernel using `evtest` when pressing the Fn
key.

Change-Id: Ie4daa64bc6b619392276d0b5f16e2d195d5bd68c
Signed-off-by: Aseda Aboagye <aaboagye@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81895
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Shelley Chen <shchen@google.com>

src/acpi/Kconfig

@@ -14,6 +14,14 @@ config ACPI_CPU_STRING
 	  Specifies the ACPI name format string used by the acpigen
 	  function to generate the processor scope. Default is CPxx.
 
+config ACPI_FNKEY_GEN_SCANCODE
+	int
+	default 0
+	help
+	  Specifies the scancode, if any, which is generated when the Fn key is
+	  pressed. A value of 0 is ignored, which is the default since most
+	  keyboards do not emit a scancode for the Fn key.
+
 config ACPI_HAVE_PCAT_8259
 	def_bool y if !ACPI_NO_PCAT_8259
 

src/acpi/acpigen_ps2_keybd.c

@@ -244,6 +244,13 @@ static void ssdt_generate_keymap(struct acpi_dp *dp, uint8_t num_top_row_keys,
 			acpi_dp_add_integer(dp_array, NULL, keymap);
 		}
 
+		/* Add the Fn-key */
+		if (CONFIG_ACPI_FNKEY_GEN_SCANCODE != 0) {
+			acpi_dp_add_integer(dp_array, NULL, KEYMAP(CONFIG_ACPI_FNKEY_GEN_SCANCODE,
+								   KEY_FN));
+			total++;
+		}
+
 		total += num_top_row_keys;
 	}