scripts/Makefile.spl: Use 'sort' in SHRUNK_ARCH_DTB rule

With configs such as "am64x_evm_a53" or "imx8mp_venice" which list
multiple device trees to build we get a warning such as:
scripts/Makefile.spl:578: target 'spl/dts/freescale/' given more than once in the same rule

If we sort this list first the warning goes away.

Tested-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Tom Rini <trini@konsulko.com>