[ARM] 3541/2: workaround for PXA27x erratum E7

Patch from Nicolas Pitre

According to the Intel PXA27x Processor Family Specification
Update document (doc.nr. 280071-009) erratum E7, some care must be taken
to locate the disabling and re-enabling of the MMU to the beginning of a
cache line to avoid problems in some circumstances.

Credits to Simon Vogl <simon.vogl@researchstudios.at> for bringing this
up.

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