mtd: fix section mismatch on sst25l

Building the kernel with 'make CONFIG_DEBUG_SECTION_MISMATCH=y´

resulted in:

WARNING: vmlinux.o(.data+0x15938): Section mismatch in reference from
the variable sst25l_driver to the function .init.text:sst25l_probe()
The variable sst25l_driver references
the function __init sst25l_probe()
If the reference is valid then annotate the
variable with __init* or __refdata (see linux/init.h) or name the variable:
*_template, *_timer, *_sht, *_ops, *_probe, *_probe_one, *_console,

Fix the section mismatch.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>