keystone2: ddr3: eliminate using global ddr3_size variable

KS2 ddr3 initialization uses ddr3_size global variable before u-boot
relocation. Even if the variable is not being used after relocation,
writing to it corrupts relocation table.

This patch removes the global ddr3_size variable and uses local one
instead.

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Tested-by: Nishanth Menon <nm@ti.com>