platform: sifive_fu740: do not use a global in da9063_reset/shutdown

da9063_reset() and da9063_shutdown() take the chip address in argument
(like similar functions), but in practice use the da9063 global struct
instead. Fix that.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Xiang W <wxjstz@126.com>