rpmsg: char: Refactor rpmsg_chrdev_eptdev_create function

rpmsg: char: Refactor rpmsg_chrdev_eptdev_create function

Introduce the rpmsg_chrdev_eptdev_alloc and rpmsg_chrdev_eptdev_add
internal function to split the allocation part from the device add.

This patch prepares the introduction of a rpmsg channel device for the
char device. An default endpoint will be created,
referenced in the rpmsg_eptdev structure before adding the devices.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220124102524.295783-9-arnaud.pouliquen@foss.st.com