soc/amd/stoneyridge: Add RO_REGION_ONLY

We only need the apu firmware in the RO region when building
for ChromeOS. Adding it to the RW regions is a waste of space.

BUG=b:70027919
TEST=Build kahlee and use cbfstool to check for "apu/amdfw" sections.

Change-Id: Ieafe4a5ec4a5e3177e4e23fcf42afa2626a0b19f
Signed-off-by: Marc Jones <marcj303@gmail.com>
Reviewed-on: https://review.coreboot.org/22766
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/amd/stoneyridge/Kconfig

@@ -357,4 +357,9 @@ config DIMM_SPD_SIZE
 	int
 	default 512	# DDR4
 
+config RO_REGION_ONLY
+	string
+	depends on CHROMEOS
+	default "apu/amdfw"
+
 endif # SOC_AMD_STONEYRIDGE_FP4 || SOC_AMD_STONEYRIDGE_FT4