rpi-firmware: modify settings for Pi 2

Remove low-level settings from config.txt like arm_freq and sdram_freq.
The defaults should be good for most and differ by Pi model.
E.g. the Pi 2 runs at 900 Mhz by default, setting arm_freq=700
like we had previously would underclock it.

Add short description in comments to options.
Mention initramfs option.

Remove obsolete options from cmdline.txt

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>