arch/csky: remove BR2_CSKY_DSP option

The DSP extention is in fact no longer used for C-SKY, nor supported
by C-SKY gcc, so we remove it.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[Thomas: split from the VDSP patch, add Config.in.legacy]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>