ARM: fix a section mismatch warning with our use of memblock

Commit 716a3dc2 (ARM: Add arm_memblock_steal() to allocate memory
away from the kernel) added a function which calls memblock_alloc().
This causes a section conflict:

WARNING: vmlinux.o(.text+0xc614): Section mismatch in reference from the function arm_memblock_steal() to the function .init.text:memblock_alloc()
The function arm_memblock_steal() references
the function __init memblock_alloc().

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>