Add CP6 enable/disable sequences to the timekeeping code and the IRQ
code.  As a result, we can't depend on CP6 access being enabled when
we enter get_irqnr_and_base anymore, so switch the latter over to
using memory-mapped accesses for now.
Signed-off-by: NLennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>