disk_access: reference count initialization calls for disks

Reference count initialization calls for disks. This changes the
behavior of the disk_access_init() function, such that disks will no
longer be initialized again if the first disk access init call
succeeds.

Disk access is reference counted in preparation for supporting disk
de-initialization, where a balanced number of disk de-initialization
calls with disk initialization calls will de-initialize the disk.

Also, remove code in disk drivers that was already checking against
duplicate disk_access_init() calls.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>