soc: nxp: lpc: lpc55xxx: fix CONFIG_LPC55XXX_USB_RAM defaults

Setting a boolean Kconfig option default to "n" after having set it to "y"
does not make the option disabled. Instead, avoid setting default to "y"
for SoCs known not to have dedicated USB RAM.

Fixes: #73912

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>