arm64: restore cpu suspend/resume functionality

Commit 4b3dc967 ("arm64: force CONFIG_SMP=y and remove redundant #ifdefs")
accidentally retained code for !CONFIG_SMP in cpu_resume function. This
resulted in the hash index being zeroed in x7 after proper computation,
which is then used to get the cpu context pointer while resuming.

This patch removes the remanant code and restores back the cpu suspend/
resume functionality.

Fixes: 4b3dc967 ("arm64: force CONFIG_SMP=y and remove redundant #ifdefs")
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>