ARM: uniphier: include <asm/system.h> instead of <asm/cache.h> from psci.c

I do not understand the change made to this file by
commit 90526e9f ("common: Drop net.h from common header").

  git show 90526e9f -- arch/arm/mach-uniphier/arm32/psci.c

It added <asm/cache.h> while this file does not call the standard cache
functions at all.

All the uniphier-specific cache functions, uniphier_cache_*() are
declared in cache-uniphier.h, which is already included from this file.

Including <asm/system.h> is sensible to fix the -Wmissing-prototypes
warnings because this file defines psci_cpu_on and psci_system_reset().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>