imx-atf: add mark quotation to prevent build issue with ccache
Otherwise, when ccache is enable the following issue occurs:
Log data follows:
| DEBUG: Executing shell function do_compile
| NOTE: make -j 8 CROSS_COMPILE=aarch64-oe-linux- PLAT=imx8qx LD=aarch64-oe-linux-ld CC=ccache aarch64-oe-linux-gcc bl31
| make: *** No rule to make target 'aarch64-oe-linux-gcc'. Stop.
| ERROR: oe_runmake failed
| WARNING: exit code 1 from a shell command.
Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
