soc: sifive: l2_cache: fix missing iounmap() in error path in sifive_l2_init()

commit 73e770f0 upstream.

Add missing iounmap() before return error from sifive_l2_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>
[conor: ccache -> l2_cache]
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>