sunxi-mali-mainline-driver: add support for building on arm64

sunxi-mali-mainline-driver: add support for building on arm64

Blobs for ARM64 are now available at Bootlin Github and
sunxi-mali-mainline package has been updated to support them.

This commit updates the <pkg>_MAKE_OPTS variable by substituting
CROSS_COMPILE and INSTALL_MOD_PATH with
LINUX_MAKE_FLAGS. LINUX_MAKE_FLAGS already includes CROSS_COMPILE and
INSTALL_MOD_PAT, but also includes the ARCH variable, which is needed
to select the appropriate architecture when building the Mali driver.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>