arch/csky: specify BR2_GCC_TARGET_FLOAT_ABI

arch/csky: specify BR2_GCC_TARGET_FLOAT_ABI

The C-SKY architecture uses two different ABIs, depending on the core
being used:

 - "abiv1" is a mcore based ISA with ELF_NUM:39 and does not support
   FPU & VDSP. It is used only for the ck610 core.

 - "abiv2" is C-SKY's own ISA with ELF_NUM:252 and supports FPU &
   VDSP. It is used for the ck807, ck810, ck860 cores.

Since "abiv1" does not support FPU, BR2_GCC_TARGET_FLOAT_ABI will
always have the value "soft" for the ck610 core.

Signed-off-by: Guo Ren <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
[Thomas: rework commit log]
Signed-off-by: Thomas Petazzoni <[email protected]>