Fix incompatible mmu-type define (#47)

Per RISC-V CPU Bindings [1], the mmu-type attribute should be set to
"riscv,sv32".

[1] https://github.com/riscv-non-isa/riscv-device-tree-doc/blob/master/bindings/riscv/cpus.txt

Author: chiangkd

Diff for: minimal.dts

@@ -25,7 +25,7 @@
             compatible = "riscv";
             reg = <0>;
             riscv,isa = "rv32ima";
-            mmu-type = "riscv,rv32";
+            mmu-type = "riscv,sv32";
             cpu0_intc: interrupt-controller {
                 #interrupt-cells = <1>;
                 #address-cells = <0>;