riscv-rv32i: Enable FPU extension only if CONFIG_FPU is enabled
riscv-rv32i: Enable FPU extension only if CONFIG_FPU is enabled

At default, CONFIG_FPU option isn't enabled. But currently,
FPU extension is always enabled when building FW image.
This will cause problem if hardware doesn't support the extension
and floating-point instructions are generated. So we fix it.

BUG=none
BRANCH=none
TEST=Floating-point instructions are used for floating point
     operation if CONFIG_FPU is enabled. Otherwise, library
     routines are used.

Signed-off-by: Dino Li <338d523ecf3700bd86eb9c34d220f91911d2a18c@ite.com.tw>
Change-Id: Ifb77bd0cca1158ca7f6637fa9ec025ac8712bbfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2227779
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Commit-Queue: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec