arm64: hibernate.c: Create a new function to handle cpu_up(sleep_cpu)

In preparation to make cpu_up/down private - move the user in arm64
hibernate.c to use a new generic function that provides what arm64
needs.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
CC: Catalin Marinas <catalin.marinas@arm.com>
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