sysreset: Add get_status method

It's useful to have the reset status of the SoC printed out during reset
(e.g. to learn whether the reset was caused by software or a watchdog).

As a first step to implement this, add a get_status method to the
sysreset class, which enables the caller to get printable information
about the reset status (akin to get_desc in the CPU uclass).

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mario Six <mario.six@gdsys.cc>