Peach-Pi: dts: add cpu-model string

Peach-Pi: dts: add cpu-model string

This platform is based on Exynos5800 but the cpu id is 0x5422.
This doesn't fit the common Exynos SoC name convention, so now,
the CPU name is defined by device tree string, to be printed
properly.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Anand Moon <linux.amoon@gmail.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>