MIPS: generic: Init command line with fw_init_cmdline()

The function bootcmdline_init() in arch/mips/kernel/setup.c will
populate the boot_command_line string using the parameters hardcoded in
the kernel, and those provided in the devicetree file. Then, it would
append the content of the arcs_cmdline variable, which is filled by the
board's plat_mem_setup() function.

The plat_mem_setup() function for the generic MIPS board would just copy
the current boot_command_line to arcs_cmdline, which is nonsense for two
reasons:
- the result will be appended to the boot_command_line anyway, so all it
  does is duplicate every single parameter on the command line;
- the code did not perform at all what it's supposed to, which is to
  retrieve the parameters passed by the bootloader.

Fix this by calling fw_init_cmdline() in plat_mem_setup(), which will
properly initialize arcs_cmdline to the parameters passed by the
bootloader.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>