nvmem: mediatek: Fix later provider initialization

Possibly, provider driver initialization is later than
consumer driver. Use function subsys_initcall to initialize
NVMEM provider early to ensure NVMEM consumer doesn't need
to -EPROBE_DEFER.

Signed-off-by: Andrew-CT Chen <andrew-ct.chen@mediatek.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>