MXS specific devices have some common infrastructure in the kernel
known as STMP devices. We have the same in barebox, but with a
mxs_ prefix instead of a stmp_ prefix. As some STMP devices are
also found on i.MX6 move the common infrastructure out of MXS
specific files and use the stmp_ prefix.

This is done in preparation for i.MX6 NAND support.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>