[POWERPC] iseries: Move e2a()/strne2a() into their only caller

The ASCII -> EBCDIC functions, e2a() and strne2a() are now only used in
dt.c, so move them in there.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>