MIPS: alchemy: provide visible function prototypes to board files

MIPS: alchemy: provide visible function prototypes to board files

Board files under arch/mips/alchemy/ define functions called from
db1xxx.c but their prototypes are only in that .c file instead of being
declared in a common header. This causes several build warnings about
missing prototypes. Provide these prototypes in a new header and include
it where necessary.

Tested-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>