hw/ppc: Take QEMU lock when calling ppc_dcr_read/write()

hw/ppc: Take QEMU lock when calling ppc_dcr_read/write()

The ppc_dcr_read() and ppc_dcr_write() functions call into callbacks
in device code, so we need to hold the QEMU iothread lock while
calling them.  This is the case already for the callsites in
kvmppc_handle_dcr_read/write(), but we must also take the lock when
calling the helpers from TCG.

This fixes a bug where attempting to initialise the PPC405EP
SDRAM will cause an assertion when sdram_map_bcr() attempts
to remap memory regions.

Reported-by: Amit Lazar <abasarlaz@hotmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200322192258.14039-1-peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>