systemtap: Disable nios2 support

systemtap: Disable nios2 support

systemtap does not support nios2 yet, so disable it.

(From OE-Core rev: 852a10e2f0b3d9fa5df8df529aac7acc424c3da9)

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Ley Foon Tan <lftan@altera.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Ross Burton <ross.burton@intel.com>
Cc: Thomas Chou <thomas@wytron.com.tw>
Cc: Walter Goossens <waltergoossens@home.nl>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>