lib: Add support for generic packing operations

This provides an unified API for accessing register bit fields
regardless of memory layout. The basic unit of data for these API
functions is the u64. The process of transforming an u64 from native CPU
encoding into the peripheral's encoding is called 'pack', and
transforming it from peripheral to native CPU encoding is 'unpack'.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>