bad_page_threshold could be configured to enable/disable the
associated bad page retirement feature in RAS.

When it's -1, ras will use typical bad page failure value to
handle bad page retirement.

When it's 0, disable bad page retirement, and no bad page
will be recorded and saved.

For other valid value, driver will use this manual value
as the threshold value of totoal bad pages.

v2: correct documentation of this parameter.

v3: remove confused statement in documentation.
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>