ARM: fix LTO for imx28_xea

When imx28_xea_defconfig is built with LTO, the compiler complains about
the two different declarations of _start:
   include/asm-generic/sections.h    as  extern void _start(void);
   arch/arm/cpu/arm926ejs/mxs/mxs.c  as  extern uint32_t _start;

Fix this.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>