ARM: bcm2835 rpi: remove hard-coded memreserve from DT

The Raspberry Pi has either 256MB or 512MB of RAM. However, a portion is
reserved for use by the VideoCore co-processor. The RPi DT contained a
/memreserve/ statement to reserve that RAM. However, the exact amount of
RAM used by the VideoCore is dynamic at boot-time; a firmware config
file specifies the amount. As such, we can't hard-code the size in the
DT. Remove the /memreserve/ statement. The bootloader is expected to
adjust the /memory properties to reflect the RAM size the ARM CPU can
use. Upstream U-Boot certainly does this, although I'm not sure that the
basic firmware does if it boots the kernel directly; users may need to
manually adjust their DT if not using U-Boot.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>