hw/char/exynos4210_uart: Constify static array and few arguments

hw/char/exynos4210_uart: Constify static array and few arguments

The static array exynos4210_uart_regs with register values is not
modified so it can be made const.

Few other functions accept driver or uart state as an argument but they
do not change it and do not cast it so this can be made const for code
safeness.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Message-id: 20170313184750.429-3-krzk@kernel.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>