memory: tegra20-emc: use DEFINE_SHOW_ATTRIBUTE to simplify code

Use DEFINE_SHOW_ATTRIBUTE helper macro to simplify the code.
No functional change.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220926130025.1061373-2-liushixin2@huawei.com