x86, earlyprintk: Move mrst early console to platform/ and fix a typo

Move the code to arch/x86/platform/mrst/. Also fix a typo to use
the correct config option: ONFIG_EARLY_PRINTK_MRST

Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: alan@linux.intel.com
LKML-Reference: <1291348298-21263-1-git-send-email-feng.tang@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>