ANDROID: mm: introduce gcma_stat_get function

Introduce helper function to get gcma stats and move enum gcma_stat_type
to include/linux/gcma.h. For debugging purposes, I'd like to print usage
status of gcma area into kernel log via show_mem vendor hook and add gcma
stats in /proc/meminfo via meminfo vendor hook.
So let's export this helper function.

Bug: 410432482
Change-Id: I1e4fd209c8a3037fba69ab2ed190f4c308477835
Signed-off-by: Sooyong Suk <s.suk@samsung.corp-partner.google.com>