ARM: OMAP2+: move MPU INTCPS, secure monitor, SDRC build directives in Makefile

Move MPU INTCPS (interrupt controller) and secure monitor code build
directives to their own Makefile sections, for clarity.  Coalesce
SDRC-related Makefile directives into the SDRC Makefile section.

Signed-off-by: Paul Walmsley <paul@pwsan.com>