ARM: brcmstb: Enable ARCH_HAS_HOLES_MEMORYMODEL

Due to the brcmstb SoC's memory map, we may have holes in the memory if
we don't populate the maximum amount of memory supported by e.g: the
first memory controller which spans either the first 1GiB or first 2GiB
of memory.

We need to select ARCH_HAS_HOLES_MEMORYMODEL in order to enable
CONFIG_HAVE_ARCH_PFN_VALID on platforms that have CONFIG_SPARSEMEM
enabled.

Signed-off-by: Danesh Petigara <danesh.petigara@broadcom.com>
[florian: Detailed commit message]
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>