tcl: target: am437x: use entire SRAM area

tcl: target: am437x: use entire SRAM area

According to AM4379 TRM [1], table 2-1 L3 Memory Map,
we have a total of 256KiB and there's no reason not to
use it all.

[1] http://www.ti.com/lit/ug/spruhl7b/spruhl7b.pdf

Change-Id: I117f2afe721bc4e3f0df304d3542e1a91aa69d12
Signed-off-by: Felipe Balbi <balbi@ti.com>
Reviewed-on: http://openocd.zylin.com/2611
Tested-by: jenkins
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>