target/ppc/translate: Add dummy implementation for dcblc instruction

target/ppc/translate: Add dummy implementation for dcblc instruction

The dcblc instruction is used by u-boot in mpc85xx/start.S. Without it,
an illegal istruction exception is generated very early in the boot
process where the processor is not yet able to handle exceptions. See:

https://github.com/u-boot/u-boot/blob/v2023.01/arch/powerpc/cpu/mpc85xx/start.S#L1840

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20230130184950.5241-1-shentey@gmail.com>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>