arm64: Slightly improve the warning on CPU0 enable-method

Commit e8765b26 (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>