include: env: ti_common: remove bootm_size

include: env: ti_common: remove bootm_size

As the size of 64b ARM kernels, DTs, vendor firmware, and initial ram
disks continues to grow, the 256MB size limit set aside for image
processing by the bootm command has become too small for some K3
reference boards.

For ARMv7 removing this limit could introduce issues so move the
bootm_size variable to ti_armv7_keystone2.env and remove the limit for
any board using a TI K3 SoC.

Signed-off-by: Bryan Brattlof <bb@ti.com>