libpayload: Add gru config

This adds a gru libpayload config, that should fit all gru-based
devices such as kevin.

As gru-based devices are CrOS devices, select the associated config
to enable CrOS-specific features.

Change-Id: I6e79b763fc497c126612b8786a669a33b57ea29f
Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-on: https://review.coreboot.org/19137
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/configs/config.gru

@@ -0,0 +1,7 @@
+CONFIG_LP_CHROMEOS=y
+CONFIG_LP_ARCH_ARM64=y
+CONFIG_LP_8250_SERIAL_CONSOLE=y
+CONFIG_LP_TIMER_RK3399=y
+CONFIG_LP_USB_EHCI=y
+CONFIG_LP_USB_OHCI=y
+CONFIG_LP_USB_XHCI=y