mailbox/drivers/hisi: Consolidate the Kconfig for the MAILBOX

The current defconfig is inconsistent as it selects the mailbox and
the clock for the hi6220 and the hi3660 without having their Kconfigs
making sure the dependencies are correct. It ends up when selecting
different versions for the kernel (for example when git bisecting)
those options disappear and they don't get back, leading to unexpected
behaviors. In our case, the cpufreq driver does no longer work because
the clock fails to initialize due to the clock stub and the mailbox
missing.

In order to have the dependencies correctly set when defaulting, let's
do the same as commit 3a49afb8 ("clk: enable hi655x common clk
automatically") where we select automatically the driver when the
parent driver is selected. With sensible defaults in place, we can leave
other choices for EXPERT.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>