firmware: stratix10-svc: build only on 64-bit ARM

The Stratix10 service layer and RCU drivers are useful only on
Stratix10, so on ARMv8.  Compile testing the RCU driver on 32-bit ARM
fails:

  drivers/firmware/stratix10-rsu.c: In function 'rsu_status_callback':
  include/linux/compiler_types.h:320:38: error: call to '__compiletime_assert_179'
    declared with attribute error: FIELD_GET: type of reg too small for mask
    _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
  ...
  drivers/firmware/stratix10-rsu.c:96:26: note: in expansion of macro 'FIELD_GET'
    priv->status.version = FIELD_GET(RSU_VERSION_MASK,

Fixes: 4483397b ("ARM: socfpga: drop ARCH_SOCFPGA")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Richard Gong <richard.gong@linux.intel.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
---
v2: add Fixes tag