phy: rockchip-emmc: adapt binding to specifiy register offset and length

The emmc-phy occupies a contiguous set of 8 registers inside the general
register files, so the reg property should specify this.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>