meson-mx-socinfo: Make local function meson_mx_socinfo_init() static

Fixes the following sparse warnings:

drivers/soc/amlogic/meson-mx-socinfo.c:107:12: warning:
 symbol 'meson_mx_socinfo_init' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>