usb: dwc2: Allow selection of data buffer size

If we use hardware with very small RAM (let's consider just a couple
of hundreds of kB but not megabytes) it is not super convenient to lose
64kB for statically allocated bufer which most probably won't be used
as big as it is. Typically we'll have much shorter data packages to
excahnge and in the worst case longer packets will be split on separate
transactions.

For those corner-cases user will be able to set his buffer size of
choice via USB_DWC2_BUFFER_SIZE option in menuconfig.

By default we'll use 64 kB as it was hard-coeded before so existing
users shouldn't be affected at all.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Marek Vasut <marex@denx.de>