arm: mxs: Add 'Wait for JTAG user' if booted in JTAG mode

When booting in JTAG mode, there is no way to use soft break-points, and
no way of knowing when SPL has finished executing (so the user can issue
a 'halt' command to load u-boot.bin for example)

Add a debug output and simple loop to stop execution at the completion of
the SPL initialisation as a pseudo break-point when booting in JTAG mode

Signed-off-by: Graeme Russ <gruss@tss-engineering.com>