mb/google/poppy/nami: Add 'detect' flag for Elan touchpad

Add the 'detect' flag to the Elan touchpad, so coreboot can determine
which touchpad type is present at runtime and generate the correct
ACPI entry for it (the Synaptics touchpad already has the flag).

Test: build/boot google/nami, verify touchpad works under Linux/Windows

Change-Id: I437d1d470552d55496dfe611f441331127c64250
Signed-off-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67306
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/mainboard/google/poppy/variants/nami/devicetree.cb

@@ -343,6 +343,7 @@ chip soc/intel/skylake
 				register "desc" = ""ELAN Touchpad""
 				register "irq" = "ACPI_IRQ_EDGE_LOW(GPP_E3_IRQ)"
 				register "wake" = "GPE0_DW2_16"
+				register "detect" = "1"
 				device i2c 15 on end
 			end
 			chip drivers/i2c/hid