gpio/xilinx: Use correct address when setting initial values.

xgpio_save_regs() is used in this driver to setup the initial
values of the registers in the hardware.

The relevant registers at that time are:
0x0 -> channel 0 data (32 bits, one for each GPIO on this channel).
0x4 -> channel 0 tri, controls in/out status for each GPIO of this channel.
0x8 -> channel 1 data
0xC -> channel 1 tri

gpio-xilinx.c defines these:
XGPIO_DATA_OFFSET   (0x0)
XGPIO_TRI_OFFSET    (0x4)
XGPIO_CHANNEL_OFFSET	0x8

Before this patch, the "data" register value of channel 1 was written
at 0x4 intead of 0x8 (overwriting the channel 0 "tri" register),
and the "tri" register value for channel 1 was written at 0x8 instead of 0xC.

Signed-off-by: Raphaël Teysseyre <rteysseyre@gmail.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>