riscv: dts: sort makefile entries by directory

New additions to the list have tried to respect alphanumeric ordering,
but the thing was out of order to start with. Sort it.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

arch/riscv/boot/dts/Makefile

 # SPDX-License-Identifier: GPL-2.0
 subdir-y += allwinner
-subdir-y += sifive
-subdir-y += starfive
-subdir-y += thead
 subdir-y += canaan
 subdir-y += microchip
 subdir-y += renesas
+subdir-y += sifive
+subdir-y += starfive
+subdir-y += thead
 
 obj-$(CONFIG_BUILTIN_DTB) := $(addsuffix /, $(subdir-y))