mailbox: tegra-hsp: use devm_kstrdup_const()

Use devm_kstrdup_const() in the tegra-hsp driver. This mostly serves as
an example of how to use this new routine to shrink driver code.

Also use devm_kzalloc() instead of regular kzalloc() to shrink the
driver even more.

Doorbell objects are only removed in the driver's remove callback so
it's safe to convert all memory allocations to devres.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>