staging: fbtft: add fb_ili9486 driver

This commit adds the fb_ili9486 driver from the fbtft project at
https://github.com/notro/fbtft

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Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Noralf Tronnes <notro@tronnes.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>