Currently, zynq-uart code is always built and has some requirements for
BSPs that use it. Instead of making all BSPs satisfy that requirement or
working around it by setting defaults, this moves the zynq-uart code
into its own spec build object so it can be included if needed.
The zynq-uart set_attributes implementation was configured to always
return false which causes spconsole01 to fail. This restores the
disabled implementation which sets the baud rate registers
appropriately and allows spconsole01 to pass. This also expands the
set_attributes functionality to allow setting of the stop bits,
character width, and parity.
