ARM: davinci: serial: remove davinci_serial_setup_clk()

Get rid of davinci_serial_setup_clk() since its not called
from multiple places now. Instead initialize clock in
davinci_serial_init() itself. This also helps get rid of
"serial_dev" member of struct davinci_soc_info.

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Suggested-by: Sekhar Nori <nsekhar@ti.com>
[nsekhar@ti.com: split removal of davinci_serial_setup_clk()
		 into a separate patch.]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>