powerpc/powernv: Document cxl dependency on special case in pnv_eeh_reset()

pnv_eeh_reset() has special handling for PEs whose primary bus is the
root bus or the bus immediately underneath the root port.

The cxl bi-modal card support added in b0b5e591

 ("cxl: Add
cxl_check_and_switch_mode() API to switch bi-modal cards") relies on
this behaviour when hot-resetting the CAPI adapter following a mode
switch.  Document this in pnv_eeh_reset() so we don't accidentally break
it.

Suggested-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>