arm64/cpufeature: Add get_arm64_ftr_reg_nowarn()

arm64/cpufeature: Add get_arm64_ftr_reg_nowarn()

There is no way to proceed when requested register could not be searched in
arm64_ftr_reg[]. Requesting for a non present register would be an error as
well. Hence lets just WARN_ON() when search fails in get_arm64_ftr_reg()
rather than checking for return value and doing a BUG_ON() instead in some
individual callers. But there are also caller instances that dont error out
when register search fails. Add a new helper get_arm64_ftr_reg_nowarn() for
such cases.

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/1590573876-19120-1-git-send-email-anshuman.khandual@arm.com
Signed-off-by: Will Deacon <will@kernel.org>