mb/clevo/l140cu: set PS/2 timeout for SeaBIOS

The keyboard only works randomly, because SeaBIOS initializes PS/2
before the EC is ready.

Set the PS/2 timeout for SeaBIOS to 500 ms, to wait for the EC before
initializing the keyboard.

Test: keyboard works fine.

Change-Id: I2be75961035f04a7ffa6f7e1dbaabb1243b857f9
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45959
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/clevo/cml-u/Kconfig

@@ -81,4 +81,9 @@ config VARIANT_DIR
 	string
 	default "l140cu" if BOARD_CLEVO_L140CU
 
+config SEABIOS_PS2_TIMEOUT
+	int
+	depends on PAYLOAD_SEABIOS
+	default 500
+
 endif