Fix kernel hang while enabling low level debug

[#1]

When to enable CONFIG_DEBUG_LL, choose the debug port and then CONFIG_EARLY_PRINTK on i.MX6, system will hang.

There is no error information there as below,

Uncompressing Linux... done, booting the kernel.
Booting Linux on physical CPU 0x0
Initializing cgroup subsys cpu
Initializing cgroup subsys cpuacct
Linux version 4.1.15-00001-gd582989-dirty (jay@jay-ubuntu) (gcc version 4.9 20
150123 (prerelease) (GCC) ) #10 SMP PREEMPT Mon Jul 17 15:08:55 CST 2017
CPU: ARMv7 Processor [412fc09a] revision 10 (ARMv7), cr=10c53c7d
CPU: PIPT / VIPT nonaliasing data cache, VIPT aliasing instruction cache
Machine model: Freescale i.MX6 Quad SABRE Smart Device Board
bootconsole [earlycon0] enabled
cma: Reserved 448 MiB at 0x2a000000
Memory policy: Data cache writealloc
-------------- hang -----------------‍‍‍‍‍‍‍‍‍‍‍‍‍

The patch fix it on android n7.1.1_1.0.0, kernel: 4.1.15.