avr32: start clocksource cleanup

Start cleaning up the AVR32 clocksource mess, starting with the cycle
counter clocksource:  remove unneeded pseudo-RTC (just inline that
call to mktime) and associated build warning, and unused sysdev.

Add comment about the problem using the cycle counter register,
and adjust the clocksource rating accordingly.  Later patches can
make this usable again (by disabling use of the idle state and
providing a proper clocksource without the weak binding hacks)
and move towards TCB-based clockevent support (including high
resolution timers) that's shared between AT91 and AVR32.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>