target/xtensa: implement NMI support

target/xtensa: implement NMI support

When NMI is configured it is taken regardless of INTENABLE SR contents,
PS.INTLEVEL or PS.EXCM. It is cleared automatically once it's taken.

Add nmi_level to XtensaConfig, puth there NMI level from the overlay or
XCHAL_NUM_INTLEVELS + 1 when NMI is not configured. Add NMI mask to
INTENABLE SR and limit CINTLEVEL to nmi_level - 1 when determining
pending IRQ level in check_interrupt(). Always take and clear pending
interrupt at nmi_level in the handle_interrupt().

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>