checkpatch: add --kconfig-prefix=CFG_

A few days before v5.9-rc1, the checkpatch.pl script was modified in
the Linux kernel tree [1]. This caused spurious warnings in the OP-TEE
CI such as [2]:

 WARNING: IS_ENABLED(CFG_VIRTUALIZATION) is normally used as IS_ENABLED(CONFIG_CFG_VIRTUALIZATION)

Fortunately, checkpatch now has an option to control the prefix used for
configuration variables [3]. Add this option to .checkpatch.conf.

Link: [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=50161266973bcc662e969e63d68fc7bff71de21b
Link: [2] https://travis-ci.org/github/OP-TEE/optee_os/builds/717905104
Link: [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3e89ad8506f39c4739a6c9ca1e1552f506f000c9
Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

.checkpatch.conf

@@ -10,3 +10,4 @@
 --ignore=NOT_UNIFIED_DIFF
 --ignore=PREFER_KERNEL_TYPES
 --ignore=USLEEP_RANGE
+--kconfig-prefix=CFG_