rewrite: arch/arm/cpu/arm926ejs/as92/dbg-serial.c

Signed-off-by: Du Huanpeng <u74147@gmail.com>
text base: change as alphascale's orignal.

this make it can be flashed with alphascale's usb tool.

Signed-off-by: Du Huanpeng <u74147@gmail.com>