hw/arm/exynos4210: Move exynos4210_init_board_irqs() into exynos4210.c

hw/arm/exynos4210: Move exynos4210_init_board_irqs() into exynos4210.c

The function exynos4210_init_board_irqs() currently lives in
exynos4210_gic.c, but it isn't really part of the exynos4210.gic
device -- it is a function that implements (some of) the wiring up of
interrupts between the SoC's GIC and combiner components.  This means
it fits better in exynos4210.c, which is the SoC-level code.  Move it
there. Similarly, exynos4210_git_irq() is used almost only in the
SoC-level code, so move it too.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220404154658.565020-8-peter.maydell@linaro.org