mtd: parsers: bcm63xx: Do not make it modular

With commit 91e81150 ("mtd: parsers: bcm63xx: simplify CFE
detection"), we generate a reference to fw_arg3 which is the fourth
firmware/command line argument on MIPS platforms. That symbol is not
exported and would cause a linking failure.

The parser is typically necessary to boot a BCM63xx-based system anyway
so having it be part of the kernel image makes sense, therefore make it
'bool' instead of 'tristate'.

Fixes: 91e81150 ("mtd: parsers: bcm63xx: simplify CFE detection")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929172726.30469-1-f.fainelli@gmail.com