arm: at91: pmc: replace the constant with a define in at91_pmc.h

To enable the clocks on the at91 boards a constant (0x4) is used.
This is replaced with a define in at91_pmc.h (1 <<  2).

Signed-off-by: Erik van Luijk <evanluijk@interact.nl>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>