mtd: m25p80: Convert to device table matching

This patch converts the m25p80 driver so that now it uses .id_table
for device matching, making it properly detect devices on OpenFirmware
platforms (prior to this patch the driver misdetected non-JEDEC chips,
seeing all chips as "m25p80").

Also, now jedec_probe() only does jedec probing, nothing else. If it
is not able to detect a chip, NULL is returned and the driver fall
backs to the information specified by the platform (platform_data, or
exact ID).

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>