[EISA] EISA registration with !CONFIG_EISA

This is a change for the EISA bus support to permit drivers to call
un/registration functions even if EISA support has not been enabled.  This is
similar to what PCI (and now TC) does and reduces the need for #ifdef clutter.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>