uboot-imx

sandbox: sysreset: Update to support power-on reset

If U-Boot is started from SPL or TPL, then those earlier phases deal with
the reset cause. On real hardware this cause may be lost once it is read.
Emulate that behaviour in sandbox by reporting a warm reset when a
previous phase has run since start-up.

Signed-off-by: Simon Glass <sjg@chromium.org>