net: dsa: mv88e6xxx: add global1 helpers

The Global (1) internal SMI device is an extended set of registers
containing ATU, PPU, VTU, STU, etc.

It is present on every switches, usually at SMI address 0x1B. But old
models such as 88E6060 access it at address 0xF, thus using REG_GLOBAL
is erroneous.

Add a global1_addr info member used by mv88e6xxx_g1_{read,write} and
mv88e6xxx_g1_wait helpers in a new global1.c file.

This patch finally removes _mv88e6xxx_reg_{read,write}, in favor on the
appropriate helpers. No functional changes here.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>