MLK-14241-1 regulator: pf1550-regulator-rpmsg: add mutex for pm_qos_add_request reentry

MLK-14241-1 regulator: pf1550-regulator-rpmsg: add mutex for pm_qos_add_request reentry

Multi drivers(mmc, cpufreq..) may access pf1550 regulator rpmsg driver at
the same time, so we have to add mutex for this multi-entry case. Otherwise
the below kernel warning maybe triggered:

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WARNING: CPU: 0 PID: 19 at kernel/power/qos.c:453 pf1550_send_message+0x4c/0xf8()
pm_qos_add_request() called for already added request
Modules linked in:
CPU: 0 PID: 19 Comm: kworker/0:1 Not tainted 4.1.33-02293-g80b8c19 #636
Hardware name: Freescale i.MX7ULP (Device Tree)
Workqueue: events od_dbs_timer
[<80015d78>] (unwind_backtrace) from [<8001271c>] (show_stack+0x10/0x14)
[<8001271c>] (show_stack) from [<8082ba50>] (dump_stack+0x88/0x9c)
[<8082ba50>] (dump_stack) from [<800387c8>] (warn_slowpath_common+0x80/0xb0)
[<800387c8>] (warn_slowpath_common) from [<80038828>] (warn_slowpath_fmt+0x30/0x40)
[<80038828>] (warn_slowpath_fmt) from [<80341180>] (pf1550_send_message+0x4c/0xf8)
[<80341180>] (pf1550_send_message) from [<80341504>] (pf1550_get_voltage+0x48/0x5c)
[<80341504>] (pf1550_get_voltage) from [<803370a0>] (_regulator_get_voltage+0x68/0xb4)
[<803370a0>] (_regulator_get_voltage) from [<8033936c>] (_regulator_do_set_voltage+0x5c/0x3e4)
[<8033936c>] (_regulator_do_set_voltage) from [<803397a4>] (regulator_set_voltage+0xb0/0x14c)
[<803397a4>] (regulator_set_voltage) from [<8058dae8>] (imx7ulp_set_target+0x178/0x238)
[<8058dae8>] (imx7ulp_set_target) from [<80584d14>] (__cpufreq_driver_target+0x164/0x294)
[<80584d14>] (__cpufreq_driver_target) from [<8058bb08>] (dbs_check_cpu+0x1a0/0x1e0)
[<8058bb08>] (dbs_check_cpu) from [<805888e8>] (od_dbs_timer+0x80/0x138)
[<805888e8>] (od_dbs_timer) from [<8004bbbc>] (process_one_work+0x118/0x3e4)
[<8004bbbc>] (process_one_work) from [<8004bed4>] (worker_thread+0x4c/0x4f4)
[<8004bed4>] (worker_thread) from [<80050e4c>] (kthread+0xdc/0xf4)
[<80050e4c>] (kthread) from [<8000f528>] (ret_from_fork+0x14/0x2c)
---[ end trace f8281ecde7a0b4ce ]---
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Signed-off-by: Robin Gong <yibin.gong@nxp.com>