soc: sifive: ccache: fix missing iounmap() in error path in sifive_ccache_init()

Add missing iounmap() before return error from sifive_ccache_init().

Fixes: a967a289

 ("RISC-V: sifive_l2_cache: Add L2 cache controller driver for SiFive SoCs")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>