y exists at KERNELBASE.  This assumption
may not be true in the case of a relocatable kernel, so use KERNELBASE
+ PHYSICAL_START to get the true beginning of usable kernel memory.

cc: Joachim Fenkes <fenkes@de.ibm.com>
cc: Christoph Raisch <raisch@de.ibm.com>
cc: Hoan-Ham Hguyen <hnguyen@de.ibm.com>
Signed-off-by: Sonny Rao <sonnyrao@us.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>