lcd: remove unaligned access in lcd_dt_simplefb_configure_node()

Some ARM compilers may emit code that makes unaligned accesses when
faced with constructs such as:

const char format[] = "r5g6b5";

Make this data static since it doesn't chagne; the compiler will simply
place it into the .rodata section directly, and avoid any unaligned run-
time initialization.

Signed-off-by: Stephen Warren <swarren@nvidia.com>