aspeed/smc: adjust the size of the register region

The SPI controller of the AST2400 SoC has less registers. So we can
adjust the size of the memory region holding the registers depending
on the controller type. We can also remove the guest_error logging
which is useless as the range of the region is strict enough.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 1483979087-32663-7-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>