ARM: plat-stmp: remove plat

Now that both users of plat-stmp have been deleted in previous patches,
delete the platform, too.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>