mb/google/rex: Enable EC_GOOGLE_CHROMEEC_BOARDID Kconfig

Enables the EC_GOOGLE_CHROMEEC_BOARDID feature so we can read
board_id() on rex.

TEST=Verified builds succeed and code is linked

Change-Id: Id202019519fc4a05c80374bc97663e59fdca3d76
Signed-off-by: Tarun Tuli <taruntuli@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66018
Reviewed-by: Subrata Banik <subratabanik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

src/mainboard/google/rex/Kconfig

@@ -1,6 +1,7 @@
 config BOARD_GOOGLE_REX_COMMON
 	def_bool n
 	select BOARD_ROMSIZE_KB_32768
+	select EC_GOOGLE_CHROMEEC_BOARDID
 	select HAVE_ACPI_RESUME
 	select HAVE_ACPI_TABLES
 	select INTEL_LPSS_UART_FOR_CONSOLE