maix-bit: add code model in target definition

This is needed to avoid linking errors because the globals are placed
in memory at address 0x80000000 which is out of bounds for the default
code model.

targets/k210.json

@@ -1,5 +1,6 @@
 {
 	"inherits": ["riscv64"],
 	"features": ["+a", "+c", "+m", "+f", "+d"],
-	"build-tags": ["k210", "kendryte"]
+	"build-tags": ["k210", "kendryte"],
+	"code-model": "medium"
 }