omap: remove omap5912osk board support

Emails to the board maintainer
"Rishi Bhattacharya <rishi@ti.com>"
have been bouncing.

Tom suggested to remove this board.

Remove also omap1510_udc.c because this is the last board
to enable it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Suggested-by: Tom Rini <trini@ti.com>