MAINTAINERS: altera-sysmgr: Fix typo in a filepath

Fix typo (s/sysgmr/sysmgr/) in the header filepath.

Fixes: f36e789a

 ("mfd: altera-sysmgr: Add SOCFPGA System Manager")
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>