powerpc/mem: Declare __flush_dcache_icache() static

__flush_dcache_icache() is only used in mem.c.

Move it before the functions that use it and declare it static.

And also fix the name of the parameter in the comment.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3fa903eb5a10b2bc7d99a8c559ffdaa05452d8e0.1617895813.git.christophe.leroy@csgroup.eu