ARM: sunxi: mc-smp: Fix "lookback" typo

The sunxi_mc_smp_lookback function is modeled after the mcpm_loopback
function in the MCPM framework. "lookback" was a typo introduced when
the new function was added.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>