Use enum for MemoryDeviceType property

The translateMemoryTypeToRedfish() function returns memory device
types as std::string literals, which lacks compile-time type
safety. This can lead to typos or invalid values that are only
caught at runtime.

Convert this function to return memory::MemoryDeviceType enum
instead. This enum is already defined in generated/enums/memory.hpp
with NLOHMANN_JSON_SERIALIZE_ENUM mapping and provides proper type
checking.

The enum automatically serializes to the same JSON strings, so
there is no functional change to the Redfish API. This provides
compile-time type checking and better error handling through the
Invalid enum value for unknown or unsupported memory types.

Tested:
- No functional change.
- Verified  MemoryDeviceType property serializes to
  identical JSON values.
- Redfish Service Validator: PASS: 6002, WARN: 353, FAIL: 0,
  NOT TESTED: 5227

Change-Id: I34087487ce88ab4767a27d93ff85448b946a6528
Signed-off-by: Vinothkumar Shanmugavel <vinothkumars@ami.com>