soc: zte: Restrict SOC_ZTE to ARCH_ZX or COMPILE_TEST

The ZTE SoC drivers are only useful when building for a ZTE ZX platform.

Fixes: 4c2c2e39 ("soc: zte: pm_domains: Prepare for supporting ARMv8 zx2967 family")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>