CONFIG_AT91_SLOW_CLOCK is not available for the default architecture x86.
Result is shown for architecture blackfin
default or selected kernelversion does not have config value CONFIG_AT91_SLOW_CLOCK.
Result is shown for kernelversion 4.0.9

Suspend-to-RAM disables main oscillator

modulename: pm_slowclock.ko
configname: CONFIG_AT91_SLOW_CLOCK

Linux Kernel Configuration
└─> Atmel AT91 System-on-Chip
└─> Suspend-to-RAM disables main oscillator

Select this if you want Suspend-to-RAM to save the most power
possible (without powering off the CPU) by disabling the PLLs
and main oscillator so that only the 32 KiHz clock is available.

When only that slow-clock is available, some peripherals lose
functionality. Many can't issue wakeup events unless faster
clocks are available. Some lose their operating state and
need to be completely re-initialized.

source code: