riscv: dts: canaan: build all devicetress if SOC_CANAAN

Testing & checking the Canaan devicetrees is inconvenient as only the
devicetree corresponding to SOC_CANAAN_K210_DTB_BUILTIN will be built.
Change the Makefile so that all devicetrees are built by default if
SOC_CANAAN but only the one specified by SOC_CANAAN_K210_DTB_BUILTIN
gets built as an object.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220705215213.1802496-14-mail@conchuod.ie
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>