xilinx: common: change bootm_size to not go beyond ram_top

xilinx: common: change bootm_size to not go beyond ram_top

The available ram can be limited by ram_top as that depends on the
reserved memory nodes provided by the device-tree (via
board_get_usable_ram_top), so make sure to respect ram_top when setting
up bootm_size to avoid overlapping reserved memory regions (e.g. memory
used by OP-TEE).

The same logic is available in env_get_bootm_size when bootm_size is
not defined by the default environment.

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Link: https://lore.kernel.org/r/20220120191730.2009270-1-ricardo@foundries.io
Signed-off-by: Michal Simek <michal.simek@xilinx.com>