phy: rockchip-emmc: fix compile issue on arm64 platform

phy: rockchip-emmc: fix compile issue on arm64 platform

This patch rename "reg" property to "reg_offset".
We rename it to fix the compile issue on ARM64 platform:
(reg_format): "reg" property in /phy has invalid length (4 bytes)
(#address-cells == 2, #size-cells == 2)

This's because "reg" is very special one which should keep the
*-cells with its parent node and can't be overwrited even if we
do that explicitly. On 32-bit plafform, the default *-cells
fit for what we assign to "reg". But that's not correct for 64-bit
platform. So we can see two possible solutions to fix this problem:
A) make phy-rockchip-emmc as a child phy node and overwrite its
parent's #address-cells and #size-cells.
B) avoid using this special property.

we use it just for passing on a offset for different Socs, and there's
no requirement to change the code to make phy-rockchip-emmc as a child
node. so choose option B) is sane.

Change-Id: Ib6a10cb8c3629ec3983854f1bfb7c2426edf79d2
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>