[MIPS] RM: fix EISA=n compilation

This patch fixes the following build error with CONFIG_EISA=n caused by
commit 231a35d3

:

<--  snip -->

...
  LD      .tmp_vmlinux1
arch/mips/sni/built-in.o: In function `snirm_a20r_setup_devinit':
a20r.c:(.init.text+0x42c): undefined reference to `sni_eisa_root_init'
a20r.c:(.init.text+0x42c): relocation truncated to fit: R_MIPS_26 against `sni_eisa_root_init'
arch/mips/sni/built-in.o: In function `snirm_setup_devinit':
rm200.c:(.init.text+0x52c): undefined reference to `sni_eisa_root_init'
rm200.c:(.init.text+0x52c): relocation truncated to fit: R_MIPS_26 against `sni_eisa_root_init'
make[1]: *** [.tmp_vmlinux1] Error 1

<--  snip  -->

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>