arm64: Slightly improve the warning on CPU0 enable-method

arm64: Slightly improve the warning on CPU0 enable-method

Commit e8765b265a69 (arm64: read enable-method for CPU0) introduced
checks for the enable method on CPU0 (to be later used with CPU
suspend). However, if the kernel is compiled for UP and a DT file is
used with a method like 'spin-table', Linux complains about 'invalid
enable method'. This patch turns it into an 'unsupported enable method'
warning.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 264666e62848c19ba9252c80e895ffec9ad1d391)
Signed-off-by: Mark Brown <broonie@linaro.org>