powerpc, 8xx: fix missing function declarations.

Add missing .h and add missing declarations in .h
Declare local functions as static.  Make interrupt_init_cpu function
signatures consistent with how decrementer_count is declared.

Based on warnings reported by 'make C=2'

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[trini: drop cpu_init_f as 8xx/83xx are different from the rest, rework
interrupt_init_cpu/decrementer_count]
Signed-off-by: Tom Rini <trini@konsulko.com>