ARM: dts: imx7s: add spba-bus abstraction

According to the i.MX 7Solo/Dual Application Processor Reference
Manual the ECSPI1/2/3, UART1/2/3 and SAI1/2/3 peripherals are
connected through the SPBA bus. Other similar SoCs such as i.MX 6UL
add this bus abstraction. This adds the bus also to the i.MX 7
device tree.

The i.MX SDMA driver uses this abstraction to configure watermark
levels slightly differently, so this might change behavior slightly.
There have no issues been observed before or after the patch.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>