ARM: at91: remove AT91_PROGRAMMABLE_CLOCKS configuration option

This AT91 specific Kconfig option removed the code that dealt with
programmable clocks. Each AT91 SoC embeds programmable clocks and
there is little gain to remove this code in case that such a clock
is not used.
If this option is not selected, it causes certain drivers to fail
to build. We simply remove this option instead of adding code just
to build a workaround.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>