i8253: Remove I8253_LOCK config

Now that all extra ifdeffed implementations are gone, depend the lock
export on PCSPKR_PLATFORM

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Russell King <linux@arm.linux.org.uk>