MIPS: Loongson32: Remove reset.c

Commit 2a31bf20

 ("watchdog: loongson1_wdt: Implement restart handler")
implmented .restart ops, Then, _machine_restart is no longer needed.
The _machine_halt and pm_power_off are also unnecessary,
which contain no hardware operations.

Therefore, remove the entire reset.c and related header file.
Update the Makefile accordingly.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>