sh: fix defconfigs for sh7751r boards

This patch fixes up the defconfig for various sh7751r based boards
by updating them to the single cpu subtype CONFIG_CPU_SUBTYPE_SH7751R.
The following sh4 boards are updated: hs7751rvoip, landisk, lboxre2,
systemh, titan.

The current defconfigs with two subtypes defined trigger a configuration
bug which result in kernel configurations with missing board support
code. We end up with kernels without board code and with generic machvec
only.

So we need this patch to make sure the board code gets compiled in.

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