imx: make imx uart work for debug mode

imx: make imx uart work for debug mode

With DEBUG_CONSOLE enabled, build will fail for imx8mq platform:

./build/imx8mq/release/bl31/imx8mq_bl31_setup.o:
In function `bl31_early_platform_setup2':
imx8mq_bl31_setup.c:(.text.bl31_early_platform_setup2+0x40):
	undefined reference to `console_uart_register'
Makefile:741: recipe for target 'build/imx8mq/release/bl31/bl31.elf' failed
make: *** [build/imx8mq/release/bl31/bl31.elf] Error 1

Besides, the .console_flush callback needs to be added to avoid
panic when debug mode is enabled, since the console_flush() will
call it without checking whether the function callback is valid.

Signed-off-by: Anson Huang <[email protected]>