Reason: usually spp uses pty as a cross-process serial data transmission channel on velaos, in the case of cross-core,
in order to maintain the universality of the solution and dynamic port support, we use rpmsgfs_mount to resolve this problem,
run this command 'mount -t rpmsgfs -o cpu=cp,fs=/dev/pts /dev/cp/pts' in rc.sysinit.ap, mount cp core's 'dev/pts' folder to
ap core's 'dev/cp/pts' folder, access 'dev/pts/x' device on ap core directly to resolve pty cross-core communication.
This solution power is abnormal on some device, pty dev as a device type on cp, but pty's property changed after mounting,
access pty as fsnode on ap core, standard fs don't support 'poll', poll pty fd will cause busyloop on ap core, because fs not support
poll and rpmsg poll is not implemented, kernel just poll_notify POLL_IN to user-process, ap core can't enter lowpower mode.
Fix: use rpmsg_uart replace rpmsgfs way on cross-core case, add new spp api bt_spp_register_app_ext and new type SPP_PORT_TYPE_RPMSG_UART,
user need tell spp server which port type, rpmsg_uart type is fixed at present and don't support dynamic port.
Signed-off-by: fangzhenwei <fangzhenwei@xiaomi.com>
chengkai