MIPS: Alchemy: Fix missing prototypes

We have a number of missing prototypes warnings for board_setup(),
alchemy_set_lpj() and prom_init_cmdline(), prom_getenv() and
prom_get_ethernet_addr(). Fix those by providing definitions for the
first two functions in au1000.h which is included everywhere relevant,
and including prom.h for the last three.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>