imx8mq_evk: Update the required ATF branch

Following the README instructions leads to a non-booting
U-Boot:

U-Boot SPL 2020.01-rc3-00070-g9a0cbae22a (Nov 25 2019 - 13:08:24 -0300)
PMIC:  PFUZE100 ID=0x10
DDRINFO: start DRAM init
DDRINFO:ddrphy calibration done
DDRINFO: ddrmix config done
Normal Boot
Trying to boot from MMC2

(It hangs here)

Use the "imx_4.19.35_1.0.0" ATF branch instead, which
fixes such problem and allow the boot to complete again.

Suggested-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Peng Fan <peng.fan@nxp.com>

board/freescale/imx8mq_evk/README

@@ -11,7 +11,7 @@ Get and Build the ARM Trusted firmware
 ======================================
 Note: srctree is U-Boot source directory
 Get ATF from: https://source.codeaurora.org/external/imx/imx-atf
-branch: imx_4.14.62_1.0.0_beta
+branch: imx_4.19.35_1.0.0
 $ make PLAT=imx8mq bl31
 $ cp build/imx8mq/release/bl31.bin $(srctree)