MIPS: ath79: Remove ar933x_uart_platform.h header

MIPS: ath79: Remove ar933x_uart_platform.h header

In commit 2a9952845155c785eb749892c0b4d6c131fe760d
(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>