Enable -Wredundant-decls warning check

This flag warns if anything is declared more than once in the same
scope, even in cases where multiple declaration is valid and changes
nothing.

Consequently, this patch also fixes the issues reported by this
flag. Consider the following two lines of code from two different source
files(bl_common.h and bl31_plat_setup.c):

IMPORT_SYM(uintptr_t, __RO_START__, BL_CODE_BASE);
IMPORT_SYM(unsigned long, __RO_START__, BL2_RO_BASE);

The IMPORT_SYM macro which actually imports a linker symbol as a C expression.
The macro defines the __RO_START__ as an extern variable twice, one for each
instance. __RO_START__ symbol is defined by the linker script to mark the start
of the Read-Only area of the memory map.

Essentially, the platform code redefines the linker symbol with a different
(relevant) name rather than using the standard symbol. A simple solution to
fix this issue in the platform code for redundant declarations warning is
to remove the second IMPORT_SYM and replace it with following assignment

static const unsigned long BL2_RO_BASE = BL_CODE_BASE;

Change-Id: If4835d1ee462d52b75e5afd2a59b64828707c5aa
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>