XMC-for-Arduino V3.5.2

What's Changed

• Add code convention.md by @LinjingZhang in #329
• Add environment setup to development instructions by @LinjingZhang in #330
• OneWire fixes by @OlafFilies in #339
• variants/XMC1400: Add SPI pin definitions. by @ederjc in #341

Full Changelog: V3.5.1...V3.5.2