vexpress/spc: fix a build warning on array bounds

With ARCH_VEXPRESS_SPC option, kernel build has the following
warning:

arch/arm/mach-vexpress/spc.c: In function ‘ve_spc_clk_init’:
arch/arm/mach-vexpress/spc.c:431:38: warning: array subscript is below array bounds [-Warray-bounds]
  struct ve_spc_opp *opps = info->opps[cluster];
                                      ^
since 'cluster' maybe '-1' in UP system. This patch does a active
checking to fix this issue.

Signed-off-by: Alex Shi <alex.shi@linaro.org>
Acked-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>