OMAP4: SPI: Fix Driver Kconfig

Change dependency to ARCH_OMAP2PLUS to allow systems based on
omap24xx, omap34xx or omap44xx

Cc: spi-devel-general@lists.sourceforge.net
Signed-off-by: Syed Rafiuddin <rafiuddin.syed@ti.com>
Signed-off-by: Abraham Arce <x0066660@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>