cmake: Add support to add symbols to nocache section

This PR allows the user to add symbols to the nocache
section. The use for this could be as follows

zephyr_linker_sources_ifdef(CONFIG_NOCACHE_MEMORY
  NOCACHE_SECTION
  nocache.ld
)

nocache.ld (as shown below) can define additional
symbols to  go into the nocache section

. = ALIGN(4);
KEEP(*(NonCacheable))

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>