Move SLEEP_MODE defines from sleep.h to device headers for xmega devices, and branch on __AVR_XMEGA__ instead of device names.

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2455 1084037a-409f-4a99-8160-1a4b3577d4c1
Move SLEEP_MODE defines from sleep.h to device headers for mega and tiny devices.

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2456 1084037a-409f-4a99-8160-1a4b3577d4c1
Remove SLEEP_MODE defines for non-xmega devices, and branch definition of set_sleep_mode on presence of SM<n> instead of device name.

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2457 1084037a-409f-4a99-8160-1a4b3577d4c1