xen/arm32: head: Setup HTTBR in enable_mmu() and add missing isb

xen/arm32: head: Setup HTTBR in enable_mmu() and add missing isb

At the moment, HTTBR is setup in create_page_tables(). This is fine as
it is called by every CPUs.

However, such assumption may not hold in the future. To make change
easier, the HTTBR is not setup in enable_mmu().

Take the opportunity to add the missing isb() to ensure the HTTBR is
seen before the MMU is turned on.

Lastly, the only use of r5 in create_page_tables() is now removed. So
the register can be removed from the clobber list of the function.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>