ARM: at91: add Shutdown Controller (SHDWC) DT support

Use a string to specific the wakeup mode to make it more readable.

Add the Real-time Clock Wake-up support too for sam9g45 and sam9x5.
Add AT91_SHDW_CPTWK0_MAX to specific the Max of the Wakeup Counter.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>