cpufreq: armada-37xx: stop using 0 as NULL pointer

stable inclusion
from stable-v5.10.166
commit 00f23016118781f85d7ffda2dccf9a43de6f1c98
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I87FRA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=00f23016118781f85d7ffda2dccf9a43de6f1c98



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[ Upstream commit 08f0adb1 ]

Use NULL for NULL pointer to fix the following sparse warning:
drivers/cpufreq/armada-37xx-cpufreq.c:448:32: sparse: warning: Using plain integer as NULL pointer

Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>