soc: ti: wkup_m3_ipc: Add debug option to halt m3 in suspend

soc: ti: wkup_m3_ipc: Add debug option to halt m3 in suspend

Add a debugfs option to allow configurable halting of the wkup_m3
during suspend at the last possible point before low power mode entry.
This condition can only be resolved through JTAG and advancing beyond
the while loop in a8_lp_ds0_handler [1]. Although this hangs the system
it forces the system to remain active once it has been entirely
configured for low power mode entry, allowing for register inspection
through JTAG to help in debugging transition errors.

Halt mode can be set using the enable_off_mode entry under wkup_m3_ipc
in the debugfs.

[1] https://git.ti.com/cgit/processor-firmware/ti-amx3-cm3-pm-firmware/tree/src/pm_services/pm_handlers.c?h=08.02.00.006#n141

Suggested-by: Brad Griffis <bgriffis@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
[dfustini: add link for a8_lp_ds0_handler() in ti-amx3-cm3-pm-firmware]
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20220502033211.1383158-1-dfustini@baylibre.com