remoteproc: Merge __rproc_boot() with rproc_boot()

The additional arguments in the internal __rproc_boot() function
were dropped in commit 2bfc311a ("remoteproc: Drop wait in
__rproc_boot()"). The exported rproc_boot() is now just a wrapper
around this internal function, so merge them together.

While at this, also remove the declaration for the previously
cleaned up rproc_boot_nowait() function.

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