sh: defconfigs for R2D-PLUS and for R2D-1

This patch removes the old r2d config file named rts7751r2d_defconfig
and adds separate config files for the two r2d board versions. The two
new defconfigs are identical with the exception of board selection:

- rts7751r2dplus_defconfig selects CONFIG_RTS7751R2D_PLUS=y
- rts7751r2d1_defconfig selects CONFIG_RTS7751R2D_1=y

Please remember to update both files with r2d changes in the future.
Signed-off-by: NMagnus Damm <damm@igel.co.jp>
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>