scripts: checkpatch: allow leading spaces in multi-lines macros

With the current clang-format rules, we have that multi-line macros that
contain empty lines (usual in driver definition macros) are formatted
with spaces only, no leading tabs. For example:

```c
 #define MYDRIVER_DEFINE(i)                 \
 	 struct mydriver_data data##i;      \
                                            \ /* starts with spaces */
	 struct mydriver_config config##i;  \
	 ...
```

This patch makes checkpatch ignore such cases, so that clang-format can
be used in tree while keeping checkpatch happy.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>