uboot-phytec

Makefile: Let mrproper remove flash.bin and flash.log

In order to generate a bootable U-Boot binary for i.MX8QXP MEK
we need to run:

$ make imx8qxp_mek_defconfig
$ make flash.bin

The resultant flash.bin and flash.log are not removed after
running 'make mrproper'.

Include these files into the CLEAN_FILES list entry so that
they can be properly deleted after 'make mrproper'.

Signed-off-by: Fabio Estevam <festevam@gmail.com>