drivers/rtc/rtc-sirfsoc.c: fix kernel panic of backing from hibernation

RTC settings will be lost if power supply is cut off after hibernation
finished, but the current "restore" function does not restore RTC related
settings, this causes rtc_read_time failure and kernel panic:

  rtc rtc0: **** DPM device timeout ****
  Stack trace:
    unwind_backtrace+0x0/0xf4
    show_stack+0x10/0x14
    dpm_wd_handler+0x24/0x28
    call_timer_fn.isra.33+0x24/0x88
    run_timer_softirq+0x178/0x1f0
    __do_softirq+0x120/0x200
    do_softirq+0x54/0x5c
    irq_exit+0x9c/0xd0
    handle_IRQ+0x44/0x90
    __irq_svc+0x40/0x70
    _raw_spin_unlock_irqrestore+0x10/0x48
    sirfsoc_rtc_iobrg_readl+0x34/0x3c
    sirfsoc_rtc_read_time+0x24/0x48
    __rtc_read_time.isra.3+0x48/0x5c
    rtc_read_time+0x30/0x44
    rtc_resume.part.9+0x20/0x104
    rtc_resume+0x5c/0x64
    dpm_run_callback.isra.4+0x2c/0x74
    device_resume+0x9c/0x144
    dpm_resume+0x100/0x224
    hibernation_snapshot+0x170/0x398
    hibernate+0x13c/0x1d8
    state_store+0xb4/0xb8
    kobj_attr_store+0x14/0x20
    sysfs_write_file+0x160/0x190
    vfs_write+0xb4/0x194
    SyS_write+0x3c/0x78

this patch uses SIMPLE_DEV_PM_OPS() to make restore() execute the
existing resume() function which will restore the set of RTC.

Signed-off-by: Xianglong Du <Xianglong.Du@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>