pistachio: Take bootargs from device tree

pistachio: Take bootargs from device tree

The boot arguments are copied into the device tree by the boot loader
and taken from the device tree by the kernel.
The code which takes the boot arguments from the different sources was
reworked with kernel 5.5.

We have to activate CONFIG_MIPS_CMDLINE_DTB_EXTEND to take the boot
arguments from the device tree.

This makes the system boot on the board again.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>