ARM: OMAP: sdrc: Fix the build break for OMAP4 only builds

ARM: OMAP: sdrc: Fix the build break for OMAP4 only builds

OMAP4 only build breaks with below error

arch/arm/mach-omap2/sdrc.c:135: error: redefinition of 'omap2_sdrc_init'
arch/arm/plat-omap/include/plat/sdrc.h:130: note: previous definition of 'omap2_sdrc_init' was here
make[1]: *** [arch/arm/mach-omap2/sdrc.o] Error 1

Fix the same by using newly introduced CONFIG_SOC_HAS_OMAP2_SDRC marco.

Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>