ARM: davinci: serial.c: remove unused serial_read_reg()

Remove function serial_read_reg() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>