nvmem: mtk-efuse: remove nvmem regmap dependency

Regmap raw accessors are bus specific implementations, using regmap raw
apis in nvmem breaks nvmem providers based on regmap mmio.
This patch moves to nvmem support in the driver to use callback
instead of regmap, which is what the nvmem core supports now.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>