arm/scp-firmware: mark as machine-specific

Firmware is machine-specific, so set PACKAGE_ARCH.

Change-Id: Ib3247d04eaacd2b4240a1e11353f06ebd381a4c6
Signed-off-by: Ross Burton <ross.burton@arm.com>

meta-arm/recipes-bsp/scp-firmware/scp-firmware_2.6.0.bb

@@ -29,6 +29,7 @@ S = "${WORKDIR}/git"
 # Allow platform specific copying of only scp or both scp & mcp, default to both
 FW_TARGETS ?= "scp mcp"
 
+PACKAGE_ARCH = "${MACHINE_ARCH}"
 COMPATIBLE_MACHINE ?= "invalid"
 
 LDFLAGS[unexport] = "1"