ARM: dts: Work around lack of cpufreq regulator lookup for exynos4210-origen and trats boards

Exynos cpufreq drivers does not support device tree based regulator
lookup, so it can get the VDD ARM regulator only by its name. To get
cpufreq working for now, this patch works this around by renaming the
regulator in board dts files to vdd_arm, which is the name expected by
the driver.

This fixes a regression introduced by dropping support of board file
based bootup of Exynos 4210 boards that rendered cpufreq inoperable on
Trats and Origen boards.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>