Replace 'Reg_array' in 'Genode::Mmio' by 'Register_array' and 'Subreg'
in 'Genode::Register', 'Genode::Mmio::Register'and
'Genode::Mmio::Register_array' by 'Bitfield'.
Update and beautify comments in the according headers and test programs.
'Reg_array' contains items whose width can be the width of the register
storage type at a max. Nethertheless they can be smaller and iterate all
subregs that are covered by the item width. The array uses as much
successive instances of its storage type as needed.
The test 'run/util_mmio' also tests these new features heavily.
The run script 'run/util_mmio.run' runs a test over basic
functionalities of 'Mmio::Register' and 'Mmio::Register::Subreg'. The
test covers the functions 'read' and 'bits', 'set', 'clear' and 'get'.
Inline function in 'Mmio::Register::Subreg' whose definition otherwise
looks ugly.
To accommodate CPU registers, which have a structured layout but don't
depend on a region base address, this patch introduces the generic
'Genode::Register' and 'Genode::Subreg' to 'register.h'.
'Mmio::Register' and 'Mmio::Subreg' inherit from them.
