arm64: dts: ti: k3-am64-main: Update TF-A's maximum size and node name

The maximum size of TF-A 2.5 has been increased to 0x1c000 [1]. In order to
account for future expansions too, increase the allocated size for TF-A to
0x20000, in the device tree node.

Also, update the node name to "tfa-sram".

[1] - https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=2fb5312f61a7de8b7a70e1639199c4f14a10b6f9



Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20210616171224.24635-2-a-govindraju@ti.com