ARM: EXYNOS: Add a alias for pdma clocks

PDMA controllers when instantiated from device tree are registered using
amba_device_register(). The registration process enables clock to the
controllers to read the peripheral id of the PDMA amba device.

In case of Exynos4, the clocks to the PDMA controllers are named as 'dma'
but amba_device_register() looks up the clock using the name 'apb_pclk'.
Hence, alias clocks with name 'apb_pclk' clock are created for clocks
with name 'dma'.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>