remoteproc/davinci: simplify the reset function

The reset_assert() function is used to make sure the DSP remote
processor is in a reset state regardless of its previous state.
The driver relies on davinci_clk_reset_{assert,deassert}()
functions for reset management which take in a clock parameter.
The assert_reset() performs a clk_get()/clk_put() cycle to
acquire the clock handle to use with this function. This is
totally unnecessary and the code can be simplified to use
the clock acquired during probe and directly use the reset
functions, so simplify this logic.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>