fix(rdn1edge): change variable type to fix gcc sign conversion error

Change variable type in function bl31_platform_setup() to fix the GCC
sign conversion error "comparison between signed and unsigned integer
expressions".

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Change-Id: Iff914bd7ad521883723c8fb34dd893412cce7fc5
1 file changed