lib/os: Add sys_multi_heap_get_heap function for multi_heap

Sometimes there's a need to perform special operations on a heap
for a specified block. I.e. release virtual memory mapping,
power off memory bank etc.
The added procedure gets a pointer to a proper heap with metadata
for a given address.

Signed-off-by: Marcin Szkudlinski <marcin.szkudlinski@intel.com>