microblaze: Add a missing single quote to make 'make help' happy

'make ARCH=microblaze help' fails with the following error due to a
missing single quote.

/bin/sh: -c: line 0: unexpected EOF while looking for matching `''
/bin/sh: -c: line 1: syntax error: unexpected end of file
make: *** [help] Error 2

Signed-off-by: Arun Bhanu <arun@bhanu.net>
Signed-off-by: Michal Simek <monstr@monstr.eu>