test: cpu: Handle both 32bit and 64bit CPUs

Handle both 32bit and 64bit systems, i.e. sandbox and sandbox64
the same way drivers/cpu/cpu_sandbox.c does, that is in case
CONFIG_PHYS_64BIT is enabled, assume 64bit address width, else
assume 32bit address width. This fixes ut_dm_dm_test_cpu test
failure on sandbox64.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>