of_serial: fix section mismatch warnings

Fix the following section mismatches:

WARNING: drivers/built-in.o(.exit.text+0x5a): Section mismatch in reference from the function of_platform_serial_exit() to the variable .devinit.data:of_platform_serial_driver
The function __exit of_platform_serial_exit() references
a variable __devinitdata of_platform_serial_driver.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>