ArmPkg/ArmLib/AArch64: Initialize the new N+1-level page table before registering it

Prior to this change, when a new page table was created at level N+1,
the reference to the table was added to the level N translation table,
before being initialized.
It means if virtual addresses were in the address range defined by
this new table the CPU would crash as the address range was not
initialized.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16206 6f19259b-4bc3-4df7-8a09-765794883524