serial: mvebu-uart: use a generic way to access the registers

There are two UART ports on Armada3700. The second UART is based on the
first one, plus additional features, but it has a different register
layout (some bit fields are also moved inside the registers).

Clearly separate register offsets and bit fields that differ between the
standard and the extended IP. Access them in a generic way. Rename the
defines with the "STD" prefix for future distinction with "EXT" defines.
Point to these defines in the main driver data structure.

The early console only uses the standard port (not extended).
Suggested-by: Wilson Ding <dingwei@marvell.com>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>