clk: Hi6220: separately build stub clock driver

The previous code, kernel builds Hi6220's common clock driver and stub
clock driver together. Stub clock driver has introduced the dependency
with CONFIG_MAILBOX, so kernel will not build Hi6220's common clock
driver due ARM64's defconfig have not enabled CONFIG_MAILBOX by default.

So separately build stub clock driver and common clock driver for
Hi6220; and only let stub clock driver has the dependency with
CONFIG_MAILBOX.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>