boot: Move framebuffer reservation to separate helper

Create separate helper for just reserving framebuffer region without
creating or enabling simple-framebuffer node.

This is useful for scenarios where user want to preserve the bootloader
splash screen till OS boots up and display server gets started without
displaying anything else in between and thus not requiring
simple-framebuffer.

Signed-off-by: Devarsh Thakkar <devarsht@ti.com>
Reviewed-by: Nikhil M Jain <n-jain1@ti.com>