[ARM] Move read of processor ID out of lookup_processor_type()

Read the processor ID at boot, and save it in "processor_id" as we
did before.  Later, when we re-parse the CPU type in the setup.c code,
re-use the value stored in "processor_id".

This allows a cleaner work-around for noMMU devices without CP#15.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>