usb: f_gsi: Memory optimization for buffers on gsi for low MEM variants

For low memory variant targets overall buffer size to be allocated
for gsi needs to be reduced. This patch introduces a defconfig
USB_LOW_MEM_VARIANT which is a boolean and will be set if target
is low memory variant and accordingly allocate memory for buffers
on GSI.

Change-Id: Ibc99fb0d04c964b3097af3896109dadb9ea473c9
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>