sh: replace sh specific CONFIG_VOYAGERGX with CONFIG_MFD_SM501

This patch replaces all instances of CONFIG_VOYAGERGX with
CONFIG_MFD_SM501. While at it we make sure the r2d code compiles
both with and without SM501.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>