hw/pci-host/bonito: Avoid buffer overrun for bad LDMA/COP accesses

The LDMA and COP memory regions represent four 32 bit registers
each, but the memory regions themselves are 0x100 bytes large.
Add guards to the read and write accessors so that bogus accesses
beyond the four defined registers don't just run off the end of
the bonldma and boncop structs and into whatever lies beyond.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>