RISC-V: Change suspend_save_csrs and suspend_restore_csrs to public function

RISC-V: Change suspend_save_csrs and suspend_restore_csrs to public function

Currently suspend_save_csrs() and suspend_restore_csrs() functions are
statically defined in the suspend.c. Change the function's attribute
to public so that the functions can be used by hibernation as well.

Signed-off-by: Sia Jee Heng <jeeheng.sia@starfivetech.com>
Reviewed-by: Ley Foon Tan <leyfoon.tan@starfivetech.com>
Reviewed-by: Mason Huo <mason.huo@starfivetech.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>