serial: bfin_uart: Adapt bf5xx serial driver to bf60x serial4 controller.

The serial4 controller on bf60x is an enhanced version of serial controller
on bf5xx. MMR size is 32 bits other than 16 bits. MMR GCTL, MCR and LCR are
combined into one control MMR. MSR and LSR are combined into one status MMR.

This patch adapts current bf5xx serial driver to serial4 controller on bf60x.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>