ARM: use MiB for vmalloc sizes

Rather than using "m" (which is the unit of metres, or milli), and
"MB" in the printk statements, use MiB to make it clear that we are
talking about the power-of-2 megabytes, aka mebibytes.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Yanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>