xtensa: Fix section mismatch between memblock_reserve and mem_reserve

Since commit 9012d011 ("compiler: allow all arches to enable
CONFIG_OPTIMIZE_INLINING"), xtensa:tinyconfig fails to build with section
mismatch errors.

WARNING: vmlinux.o(.text.unlikely+0x68): Section mismatch in reference
	from the function ___pa()
	to the function .meminit.text:memblock_reserve()
WARNING: vmlinux.o(.text.unlikely+0x74): Section mismatch in reference
	from the function mem_reserve()
	to the function .meminit.text:memblock_reserve()
FATAL: modpost: Section mismatches detected.

This was not seen prior to the above mentioned commit because mem_reserve()
was always inlined.

Mark mem_reserve(() as __init_memblock to have it reside in the same
section as memblock_reserve().

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Message-Id: <1559220098-9955-1-git-send-email-linux@roeck-us.net>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>