MIPS: ath79: Remove ar933x_uart_platform.h header

In commit 15ef17f6


(tty: ar933x_uart: use the clk API to get the uart
clock), the AR933x UART driver for has been converted
to get the uart clock rate via the clock API and it
does not use the platform data anymore.

Remove the ar933x_uart_platform.h header file and get
rid of the superfluous variable and initialization code
in platform setup.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5832/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>