RISC-V: fix incorrect type of ARCH_CANAAN_K210_DTB_SOURCE

When adding the ARCH_ symbol for the builtin K210 dtb, I erroneously
used the bool type for something that is quite obviously a string.
Unfortunately, there is no such thing as "def_string", but in this case
we can use "default" to propagate the value of
SOC_CANAAN_K210_DTB_SOURCE to ARCH_CANAAN_K210_DTB_SOURCE.
ARCH_CANAAN_K210_DTB_SOURCE is not user selectable, so using
olddefconfig etc will update ARCH_CANAAN_K210_DTB_SOURCE to reflect any
changes made to SOC_CANAAN_K210_DTB_SOURCE.

Fixes: fc432119 ("RISC-V: kconfig.socs: convert usage of SOC_CANAAN to ARCH_CANAAN")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/all/20230111104848.2088516-1-conor.dooley@microchip.com/Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig.socs

@@ -87,7 +87,8 @@ config SOC_CANAAN_K210_DTB_BUILTIN
 	  If unsure, say Y.
 
 config ARCH_CANAAN_K210_DTB_SOURCE
-	def_bool SOC_CANAAN_K210_DTB_SOURCE
+	string
+	default SOC_CANAAN_K210_DTB_SOURCE
 
 config SOC_CANAAN_K210_DTB_SOURCE
 	string "Source file for the Canaan Kendryte K210 builtin DTB"