cxl: Add image control to sysfs

load_image_on_perst identifies whether a PERST will cause the image to be
flashed to the card. And if so, which image.

Valid entries are: "none", "user" and "factory".

A value of "none" means PERST will not cause the image to be flashed. A
power cycle to the pcie slot is required to load the image.

"user" loads the user provided image and "factory" loads the factory image upon
PERST.

sysfs updates the cxl struct in the driver then calls cxl_update_image_control
to write the vals in the VSEC.

Signed-off-by: Ryan Grimm <grimm@linux.vnet.ibm.com>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>