hw/ppc: Fix build error with CONFIG_POWERNV disabled

hw/ppc: Fix build error with CONFIG_POWERNV disabled

Currently when CONFIG_POWERNV is not enabled, the build fails, such as
with --without-default-devices:

    $ ./configure --without-default-devices
    $ make

    [281/283] Linking target qemu-system-ppc64
    FAILED: qemu-system-ppc64
    cc -m64 @qemu-system-ppc64.rsp
    /usr/bin/ld: libqemu-ppc64-softmmu.a.p/target_ppc_misc_helper.c.o: in function `helper_load_sprd':
    .../target/ppc/misc_helper.c:335:(.text+0xcdc): undefined reference to `pnv_chip_find_core'
    /usr/bin/ld: libqemu-ppc64-softmmu.a.p/target_ppc_misc_helper.c.o: in function `helper_store_sprd':
    .../target/ppc/misc_helper.c:375:(.text+0xdf4): undefined reference to `pnv_chip_find_core'
    collect2: error: ld returned 1 exit status
    ...

This is since target/ppc/misc_helper.c references PowerNV specific
'pnv_chip_find_core' call.

Split the PowerNV specific SPRD code out of the generic PowerPC code, by
moving the SPRD code to pnv.c

Fixes: 9808ce6d5cb ("target/ppc: Big-core scratch register fix")
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Reported-by: Thomas Huth <thuth@redhat.com>
Suggested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
Acked-by: Cédric Le Goater <clg@redhat.com>
Message-ID: <20250820122516.949766-2-adityag@linux.ibm.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>