mx28evk: README: Revision C is also supported

Adjust the text to mention that rev C of the board is also supported.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>

board/freescale/mx28evk/README

 FREESCALE MX28EVK
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-Supported hardware: only MX28EVK rev D is supported in U-boot.
+Supported hardware: MX28EVK rev C and D are supported in U-boot.
 
 Files of the MX28EVK port
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