soc: spcom: fix race during spcom channel create

spcom_register_rpmsg_drv() can hit race from different contexts:
when create channel occurs for uefi loaded sp application
and spdaemon uses the cryptoapp channel early, and at the same
time sec_nvm channel also creates new channel.

This change locks globally access to the channel table for
spcom_create_channel_chardev() command.

Change-Id: Ic4daee06591427146602f41227e77b3ccc3f3151
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>