RTC: sa1100: Update the sa1100 RTC driver.

Since PIE interrupts are now emulated, this patch removes the previous
code that used the hardware counters.

The removal of read_callback() also fixes a wrong user space behaviour
of this driver, which was not returning the right value to read().

[john.stultz: Merge fixups]

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Alessandro Zummo <a.zummo@towertech.it>
CC: Marcelo Roberto Jimenez <mroberto@cpti.cetuc.puc-rio.br>
CC: rtc-linux@googlegroups.com
Signed-off-by: Marcelo Roberto Jimenez <mroberto@cpti.cetuc.puc-rio.br>
Signed-off-by: John Stultz <john.stultz@linaro.org>