ns16550: add support for UART parameters to be specifed with name-value pairs

ns16550: add support for UART parameters to be specifed with name-value pairs

Add name=value parsing options for com1 and com2 to add flexibility
in setting register values for MMIO UART devices.

Maintain backward compatibility with previous positional parameter
specfications.

eg. com1=115200,8n1,0x3f8,4
eg. com1=115200,8n1,0x3f8,4,reg_width=4,reg_shift=2
eg. com1=baud=115200,parity=n,reg_width=4,reg_shift=2,irq=4

Signed-off-by: Swapnil Paratey <swapnil.paratey@amd.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>