xen/arm: lpae: Rename LPAE_ENTRIES_MASK_GS to LPAE_ENTRY_MASK_GS

xen/arm: lpae: Rename LPAE_ENTRIES_MASK_GS to LPAE_ENTRY_MASK_GS

Commit 05031fa87357 "xen/arm: guest_walk: Only generate necessary
offsets/masks" introduced LPAE_ENTRIES_MASK_GS. In a follow-up patch,
we will use it to define LPAE_ENTRY_MASK.

This will lead to inconsistent naming. As LPAE_ENTRY_MASK is used in
many places, it is better to rename LPAE_ENTRIES_MASK_GS and avoid
some churn.

So rename LPAE_ENTRIES_MASK_GS to LPAE_ENTRY_MASK_GS.

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