The existing mips_io_port_base variable isn't suitable for use early
during boot since it will be stored in the .data section which may not
be writable pre-relocation. Fix this by moving the I/O port base address
into struct arch_global_data. In order to avoid adding this field for
all targets, make this dependant upon a new Kconfig entry
CONFIG_DYNAMIC_IO_PORT_BASE. Malta is the only board which sets a
non-zero I/O port base, so select this option only for Malta.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>

Move the definition of this variable out of arch/mips/lib/board.c in
preparation for allowing use of generic board on MIPS, which will lead
to this file not being compiled.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>