cxl: Add CONFIG_CXL_EEH symbol

CONFIG_CXL_EEH is for CXL's EEH related code.

Other drivers can depend on or #ifdef on this symbol to configure
PERST behaviour, allowing CXL to participate in the EEH process.

Reviewed-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>