iommu: spapr_tce: Disable compile testing to fix build on book3s_32 config

Although SPAPR_TCE_IOMMU itself can be compile tested on certain PowerPC
configurations, its presence makes arch/powerpc/kvm/Makefile to select
modules which do not build in such configuration.

The arch/powerpc/kvm/ modules use kvm_arch.spapr_tce_tables which exists
only with CONFIG_PPC_BOOK3S_64.  However these modules are selected when
COMPILE_TEST and SPAPR_TCE_IOMMU are chosen leading to build failures:

    In file included from arch/powerpc/include/asm/book3s/64/mmu-hash.h:20:0,
                     from arch/powerpc/kvm/book3s_64_vio_hv.c:22:
    arch/powerpc/include/asm/book3s/64/pgtable.h:17:0: error: "_PAGE_EXEC" redefined [-Werror]
     #define _PAGE_EXEC  0x00001 /* execute permission */

    In file included from arch/powerpc/include/asm/book3s/32/pgtable.h:8:0,
                     from arch/powerpc/include/asm/book3s/pgtable.h:8,
                     from arch/powerpc/include/asm/pgtable.h:18,
                     from include/linux/mm.h:95,
                     from arch/powerpc/include/asm/io.h:29,
                     from include/linux/io.h:13,
                     from include/linux/irq.h:20,
                     from arch/powerpc/include/asm/hardirq.h:6,
                     from include/linux/hardirq.h:9,
                     from include/linux/kvm_host.h:7,
                     from arch/powerpc/kvm/book3s_64_vio_hv.c:12:
    arch/powerpc/include/asm/book3s/32/hash.h:29:0: note: this is the location of the previous definition
     #define _PAGE_EXEC 0x200 /* software: exec allowed */

Fixes: e93a1695 ("iommu: Enable compile testing for some of drivers")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200414142630.21153-1-krzk@kernel.org


Signed-off-by: Joerg Roedel <jroedel@suse.de>