Clean up omap24xx.h symbolic constants

These patches clean up the symbolic constants defined in include/asm-arm/arch-omap/omap24xx.h. The names of some constants, like OMAP24XX_SDRC_BASE, denote that their values are the same on all OMAP24XX platforms. This isn't so - the constant is defined to different values depending on the Kconfig architecture parameters. Also, we'd eventually like to create kernels that can be used on both 2420 or 2430, with runtime CPU detection. These types of defines prevent that from happening.

Fix these defines by explicitly defining separate constants for both architectures, when the values are different. Use ifdefs to control which value is assigned to the "base name" of the constant.

So, to continue the example above, the OMAP24XX_SDRC_BASE constant will be removed, and replaced with two defines: OMAP2420_SDRC_BASE and OMAP243X_SDRC_BASE. Depending on which CONFIG_ARCH_OMAP* Kconfig parameter is specified, we then set an SDRC_BASE define to point to one of the two architecture-specific values.

These patches are necessary prerequisites for the forthcoming PRCM cleanup series.

- Paul