ARM: riscpc: parse video information from tagged list

Correctly parse the video information from the tagged list, so that we
end up with the right bytes-per-char values.  When booting with a
tagged list rather than a param block, this allows the decompressor to
display its messages during boot on the screen.

(Boot loaders normally pass a param block on this platform, but the
latest boot loader version recently released does not.)

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>