ARM: use "* SZ_1M" rather than "<< 20"

Make the default vmalloc size clearer by using a more natural
multiplication by SZ_1M rather than a shift left by 20 bits.

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>