spi/bcm63xx-hsspi: add support for dual spi read/write

Add support for dual read/writes on spi-bcm63xx-hsspi. This has been
tested with a s25fl129p1 dual read capable spi flash, with a nice speed
improvement:

serial read:

root@OpenWrt:/# time dd if=/dev/mtd4 of=/dev/null bs=8192
2032+0 records in
2032+0 records out
real    0m 4.39s
user    0m 0.00s
sys     0m 1.55s

dual read:

root@OpenWrt:/# time dd if=/dev/mtd4 of=/dev/null bs=8192
2032+0 records in
2032+0 records out
real    0m 3.09s
user    0m 0.00s
sys     0m 1.56s

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Mark Brown <broonie@kernel.org>