fusee_cpp: implement erista pll selection logic for mtc

libraries/libvapours/include/vapours/reg.hpp

@@ -56,6 +56,12 @@ namespace ams::reg {
         return (EncodeMask(masks) | ...);
     }
 
+    template<typename IntType> requires UnsignedNonConstIntegral<IntType>
+    constexpr ALWAYS_INLINE IntType GetField(const IntType &value, const BitsMask mask) { return (value & EncodeMask(mask)) >> GetOffset(mask); }
+
+    template<typename IntType> requires UnsignedNonConstIntegral<IntType>
+    constexpr ALWAYS_INLINE void SetField(IntType &value, const BitsValue v) { value = (value & ~EncodeMask(v)) | EncodeValue(v); }
+
     template<typename IntType> requires UnsignedNonConstIntegral<IntType>
     ALWAYS_INLINE void Write(volatile IntType *reg, std::type_identity_t<IntType> val) { *reg = val; }