microblaze: switch to NO_BOOTMEM

Microblaze doesn't set CONFIG_NO_BOOTMEM and so memblock_virt_alloc()
doesn't work for CONFIG_HAVE_MEMBLOCK && !CONFIG_NO_BOOTMEM.

Similar change was already done by others architectures
"ARM: mm: Remove bootmem code and switch to NO_BOOTMEM"
(sha1: 84f452b1)
or
"openrisc: Consolidate setup to use memblock instead of bootmem"
(sha1: 266c7fad)
or
"parisc: Drop bootmem and switch to memblock"
(sha1: 4fe9e1d9)
or
"powerpc: Remove bootmem allocator"
(sha1: 10239733)
or
"s390/mm: Convert bootmem to memblock"
(sha1: 50be6345)
or
"sparc64: Convert over to NO_BOOTMEM."
(sha1: 625d693e)
or
"xtensa: drop sysmem and switch to memblock"
(sha1: 0e46c111)

Issue was introduced by:
"of/fdt: use memblock_virt_alloc for early alloc"
(sha1: 0fa1c579

)

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Alvaro Gamez Machado <alvaro.gamez@hazent.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>