I found the solution. I'm posting it for anyone who might need it.
1) Mount SD card, which have Android image, to your PC
2) Open console and mount rootfs partition
3) open init.rc with nano command
nano init.rc
4) There are shell commands, which run in boot section. Add following shell commands to somewhere of your choice:
chmod 0666 /sys/kernel/debug/omap_mux/uart1_rxd
write 20 /sys/kernel/debug/omap_mux/uart1_rxd 20
chmod 0666 /sys/kernel/debug/omap_mux/uart1_txd
write 0 /sys/kernel/debug/omap_mux/uart1_txd 0
5) Save file and boot your Beaglebone with the SD card.
Now you have permision to read/write to /sys/kernel/debug/omap_mux/uart1_txd and /sys/kernel/debug/omap_mux/uart1_rxd. And their modes are set to MODE0.