x86: sysreset: Separate out the EFI code

The EFI implementation of reset sits inside the driver and is called
directly from outside the driver, breaking the normal driver-model
conventions. Worse, it passed NULL as the device pointer, hoping that
the called function won't use it, which breaks as soon as code is added
to use it.

Separate out the implementation to improve the situation enough to allow
a future patch to add new sysreset features.

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