pass: Rename governor module to pass-gov and add governor helper functions

pass: Rename governor module to pass-gov and add governor helper functions

This patch renames the governor module's filename from pass-core.c
to pass-gov.c beceause pass-core.c includes the governor functions.

And it adds new following governor functions. On previous, other module
,such as pass.c, access the field of struct pass_governor direclty without
any wrapper function.
- int pass_governor_init(struct pass_policy *);
- int pass_governor_exit(struct pass_policy *);
- int pass_governor_update(struct pass_policy *, enum pass_gov_state);

Change-Id: I772edc602325409faf0b747cc741be5d83151c23
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>