sunxi: Avoid duplicate reset_cpu with SYSRESET enabled

The sysreset uclass unconditionally provides a definition of the
reset_cpu() function. So does the sunxi board code. Fix the build with
SYSRESET enabled by omitting the function from the board code in that
case. The code still needs to be kept around for use in SPL.

Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Stefan Roese <sr@denx.de>