imx: imx7d: fix ahb clock mux 1

The clock parent of the AHB root clock when using mux option 1
is the SYS PLL 270MHz clock. This is specified in  Table 5-11
Clock Root Table of the i.MX 7Dual Applications Processor
Reference Manual.

While it could be a documentation error, the 270MHz parent is
also mentioned in the boot ROM configuration in Table 6-28: The
clock is by default at 135MHz due to a POST_PODF value of 1
(=> divider of 2).

Signed-off-by: Stefan Agner <stefan@agner.ch>