ARM: OMAP2+: fix build breakage introduced by commit b7754452

Commit b7754452 ("mtd: onenand: omap:
use pdata info instead of cpu_is") broke an OMAP3+4 build and an N800
multi-OMAP2xxx build here:

drivers/built-in.o: In function `omap2_onenand_probe':
drivers/mtd/onenand/omap2.c:742: undefined reference to `omap2_onenand_read_bufferram'
drivers/mtd/onenand/omap2.c:743: undefined reference to `omap2_onenand_write_bufferram'
drivers/mtd/onenand/omap2.c:742: undefined reference to `omap2_onenand_read_bufferram'
drivers/mtd/onenand/omap2.c:743: undefined reference to `omap2_onenand_write_bufferram'

...

drivers/built-in.o: In function `omap2_onenand_probe':
drivers/mtd/onenand/omap2.c:788: undefined reference to `omap3_onenand_read_bufferram'
drivers/mtd/onenand/omap2.c:788: undefined reference to `omap3_onenand_write_bufferram'

Fix by declaring static functions for the missing symbols, rather than
just prototypes.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Afzal Mohammed <afzal@ti.com>
Cc: Tony Lindgren <tony@atomide.com>