xlat: Fix checks in mmap_add() and mmap_add_ctx()

xlat: Fix checks in mmap_add() and mmap_add_ctx()

Commit 79621f0038b789de23ecc8891024f7cf6aa65999 broke sgi575.

It is possible to have a region with 0 as value for the attributes. It
means device memory, read only, secure, executable. This is legitimate
if the code is in flash and the code is executed from there.

This is the case for SGI_MAP_FLASH0_RO, defined in the file
plat/arm/css/sgi/sgi_plat.c.

This problem is solved by checking both size and attributes in xlat v1.
In xlat v2, it is enough to check the granularity, as it can never be 0.

Change-Id: I7be11f1b0e51c4c2ffd560b4a6cdfbf15de2c276
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>