xen/arm32: head: Introduce get_table_slot() and use it

xen/arm32: head: Introduce get_table_slot() and use it

There are a few places in the code that need to find the slot at a
given page-table level.

So create a new macro get_table_slot() for that. This will reduce
the effort to figure out whether the code is doing the right thing.

The new macro is using 'ubfx' (or 'lsr' for the first level) rather
than the existing sequence (mov_w, lsr, and) because it doesn't require
a scratch register and reduce the number of instructions (4 -> 1).

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Tested-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Wei Chen <Wei.Chen@arm.com>