mips: suspend: include linux/suspend.h as needed

mips: suspend: include linux/suspend.h as needed

A couple of functions are defined by the architecture and declared in
linux/suspend.h, but mips is lacking the corresponding #include statement
before the definition:

arch/mips/power/cpu.c:16:6: warning: no previous prototype for 'save_processor_state' [-Wmissing-prototypes]
arch/mips/power/cpu.c:26:6: warning: no previous prototype for 'restore_processor_state' [-Wmissing-prototypes]
arch/mips/power/cpu.c:36:5: warning: no previous prototype for 'pfn_is_nosave' [-Wmissing-prototypes]
arch/mips/power/hibernate.c:6:5: warning: no previous prototype for 'swsusp_arch_resume' [-Wmissing-prototypes]

Link: https://lkml.kernel.org/r/20231204115710.2247097-18-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stephen Rothwell <sfr@rothwell.id.au>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>