remoteproc/davinci: prepare and unprepare the clock where needed

We're currently switching the platform to using the common clock
framework. We need to explicitly prepare and unprepare the rproc
clock.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Suman Anna <s-anna@ti.com>
Reviewed-by: David Lechner <david@lechnology.com>
Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>