dts: arm: nxp: rt1015: correct FlexRAM bank allocation

Although the RT1015 only supports 128 KB of FlexRAM being used at once, the
default fusemap overallocates 160KB of FlexRAM. The JLink flashloader
algorithm appears to rely on the 64KB of DTCM in the default fusemap
being configured. Reducing the DTCM allocation resulted in JLink
failing to flash the SOC.

To resolve this, utilize the default fusemap of {O, O, D, D, I} for the
RT1015 FlexRAM setup. Add a note about the restrictions on using
overallocated FlexRAM to the SOC DTSI.

Fixes #65889

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>