xen/arm: debug-exynos4210: Remove early_uart_init

xen/arm: debug-exynos4210: Remove early_uart_init

The function early_uart_init contains specific initialization for the
Arndale Board 5250.

Usually U-boot as already setup the UART correctly (ie. clock, baud rate...)
so we don't have to do again.

Futhermore, this code won't work on new platform such as the Arndale Octa.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Cc: HyonYoung Choi <commani@gmail.com>
Cc: Meng Xu <xumengpanda@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>