xen/arm: Tweak the dump page-table walk output

xen/arm: Tweak the dump page-table walk output

Currently the output is looking like:

(XEN) 1ST[0x1] = 0x000000004015ff7f
(XEN) 2ND[0x1f] = 0x00500000bfe00f7d

The content of the entries are not aligned making a bit trickier to
read (I appreciate this is a matter of taste).

Align the values by forcing the index to be always printed using
3 characters (enough to cover 512 in hexadecimal).

New output:

(XEN) 1ST[0x001] = 0x000000004015ff7f
(XEN) 2ND[0x01f] = 0x00500000bfe00f7d

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