nvmem: core: add nvmem_cell_read_common

Now there are nvmem_cell_read_u16 and nvmem_cell_read_u32.
They are very similar, let's strip out a common part.

And use nvmem_cell_read_common to simplify their implementation.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20200310132257.23358-3-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>