zynqmp: Run board_get_usable_ram_top() only on main U-Boot

With commit ce39ee28 ("zynqmp: Do not place u-boot to reserved memory
location"), the function board_get_usable_ram_top() is allocating
MMU_SECTION_SIZE of about 2MB using lmb_alloc(). But we dont have this
much memory in case of mini U-Boot.

Keep these functions which use lmb under CONFIG_LMB so that they are
compiled and used only when LMB is enabled.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/75e52def75f573e554a6b177a78504c128cb0c4a.1657183534.git.michal.simek@amd.com