ARM: riscpc: Fix fall-through warning for Clang

Fix the following fallthrough warning:

arch/arm/mach-rpc/riscpc.c:52:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
           default:
           ^
arch/arm/mach-rpc/riscpc.c:52:2: note: insert 'break;' to avoid fall-through
           default:
           ^
           break;

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/202107260355.bF00i5bi-lkp@intel.com/


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>