Update README.MD to include the Turing Pi V2.5.2 SPI NAND information

[dgdonovan]

Please update README.MD to modify and add the following information to the Turing Pi 2 hardware description section:

• 128 MB SPI NAND flash (MX35LF1GE4AB), V2.3/V2.4
• 256 MB SPI NAND flash (MX35LF2GE4AD), V2.5.2