usb: ums: move ums code from trats to Samsung common directory

UMS init was implemented in trats board file but mostly it comprises
common code. Due to that it has been moved to common/ums.c to avoid
code duplication in the future.

Changes:
- move ums initialization code from trats to common/ums.c
- remove unused CONFIG_USB_GADGET_MASS_STORAGE from trats.h

Changes v2:
- move this patch at the top of code cleanups patches

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Minkyu Kang <mk7.kang@samsung.com>