fix(brcm): fix misspelled header inclusion guard

The header inclusion guard for some header for the Broadcom Stingray
board was misspelled.

Make the preprocessor symbol for the #ifndef and #define lines the
same, so that the double inclusion protection works as expected.

Change-Id: I19d73c854cd0689a248ce914ef35ae87c39ebf39
Signed-off-by: Andre Przywara <andre.przywara@arm.com>