ARM: Remove deprecated symbol from defconfig files

Commit b2b3a8b9 ("power/reset: Remove sun6i reboot driver") removed
the sun6i reboot driver. But sunxi_defconfig and multi_v7_defconfig
still contain the symbol CONFIG_POWER_RESET_SUN6I that was deprecated
by that commit, so remove it.

Signed-off-by: Timo Sigurdsson <public_timo.s@silentcreek.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>