net: lpc32xx: Use IRAM for transmit buffer

Since the introduction of the driver, some memory in IRAM is reserved for
the TX buffers.

However there are not used but instead of it, it is the buffer provided
by the net stack which is used. As stated in the comment of the driver,
not using the IRAM buffer could cause cache issue and lower the
throughput.

For the second argument it is less the case for transmitting buffers
because the throughput gain in IRAM is mitigated by the time to copy the
data from RAM to IRAM, but the first argument is still valid and indeed
this patch fixes issue seen with Ethernet on some boards

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>