soc: a64fx-diag: disable modular build

soc: a64fx-diag: disable modular build

The NMI infrastructure cannot currently be used in loadable modules:

ERROR: modpost: "free_nmi" [drivers/soc/fujitsu/a64fx-diag.ko] undefined!
ERROR: modpost: "request_nmi" [drivers/soc/fujitsu/a64fx-diag.ko] undefined!
ERROR: modpost: "enable_nmi" [drivers/soc/fujitsu/a64fx-diag.ko] undefined!

Disable this for now to make allmodconfig build again. We may revisit
this and export those symbols instead in the future.

Reported-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>