ARM: add psci_arch_init() declaration for CONFIG_ARMV7_PSCI

arch/arm/include/asm/system.h declares psci_arch_init(), but it is
surrounded by #ifdef CONFIG_ARMV8_PSCI.

psci_arch_init() is called for CONFIG_ARMV7_PSCI too. Add the missing
function declaration.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>