avr32: use syscall prototypes from asm-generic instead of arch

This patch removes the redundant syscalls prototypes in the architecture
specific syscalls.h header file. These were identical with the ones in
asm-generic/syscalls.h.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Reported-by: Peter Huewe <PeterHuewe@gmx.de>
Reported-by: Sven Schnelle <svens@stackframe.org>
Cc: stable <stable@kernel.org>