ram: k3-j721e: rename BIT_MASK()

The macro BIT_MASK is already defined in include/linux/bitops.h. To avoid
name collisions rename BIT_MASK() in drivers/ram/k3-j721e/lpddr4_private.h
to LPDDR4_BIT_MASK().

Remove superfluous parantheses.
Remove superfluous comparison to 0.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>