[dwgpio] Use fdt_reg() to get GPIO port numbers

[dwgpio] Use fdt_reg() to get GPIO port numbers

DesignWare GPIO port numbers are represented as unsized single-entry
regions.  Use fdt_reg() to obtain the GPIO port number, rather than
requiring access to a region cell size specification stored in the
port group structure.

This allows the field name "regs" in the port group structure to be
repurposed to hold the I/O register base address, which then matches
the common usage in other drivers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>