x86: sysreset: Implement the get_last() method

Add a default implementation of this method which always indicates that
the last reset was a power-on reset. This is the most likely type of reset
and without a PCH-specific driver we cannot determine any other type.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>