ppc405_boards: Don't size flash memory to match backing image

ppc405_boards: Don't size flash memory to match backing image

Machine "ref405ep" maps its flash memory at address 2^32 - image size.
Image size is rounded up to the next multiple of 64KiB.  Useless,
because pflash_cfi02_realize() fails with "failed to read the initial
flash content" unless the rounding is a no-op.

If the image size exceeds 0x80000 Bytes, we overlap first SRAM, then
other stuff.  No idea how that would play out, but useful outcomes
seem unlikely.

Map the flash memory at fixed address 0xFFF80000 with size 512KiB,
regardless of image size, to match the physical hardware.

Machine "taihu" maps its boot flash memory similarly.  The code even
has a comment /* XXX: should check that size is 2MB */, followed by
disabled code to adjust the size to 2MiB regardless of image size.

Its code to map its application flash memory looks the same, except
there the XXX comment asks for 32MiB, and the code to adjust the size
isn't disabled.  Note that pflash_cfi02_realize() fails with "failed
to read the initial flash content" for images smaller than 32MiB.

Map the boot flash memory at fixed address 0xFFE00000 with size 2MiB,
to match the physical hardware.  Delete dead code from application
flash mapping, and simplify some.

Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20190308094610.21210-9-armbru@redhat.com>