Add new sysfs interface to shows the status of psp vbflash status.

V2: rename the sysfs interface, and set more return value.
    (0: not start; 1: in progress; MBX115 value when vbflash finish)
V3: warning fixes
Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>