riscv: Build loader.bin exclusively for Canaan K210

riscv: Build loader.bin exclusively for Canaan K210

According to the explanation in commit ef10bdf9c3e6 ("riscv:
Kconfig.socs: Split ARCH_CANAAN and SOC_CANAAN_K210"),
loader.bin is a special feature of the Canaan K210 and
is not applicable to other SoCs.

Fixes: e79dfcbfb902 ("riscv: make image compression configurable")
Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Link: https://lore.kernel.org/r/20251029094429.553842-1-jiangfeng@kylinos.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>