ppc440_sdram: QOM'ify

ppc440_sdram: QOM'ify

Change the ppc440_sdram model to a QOM class derived from the
PPC4xx-dcr-device and name it ppc4xx-sdram-ddr2. This is mostly
modelling the DDR2 SDRAM controller found in the 460EX (used on the
sam460ex board). Newer SoCs (regardless of their PPC core, e.g. 405EX)
may have this controller but we only emulate enough of it for the
sam460ex u-boot firmware.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <3e82ae575c7c41e464a0082d55ecb4ebcc4d4329.1664021647.git.balaton@eik.bme.hu>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>