ARM: atmel: spl: make css field configurable

The clock source for master clock can be slow clock, main clock,
plla clock or upll clock. So, make the clock source selection
field in mckr can be configured.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>