fbtft: Describe function parameters in kernel-doc

fbtft: Describe function parameters in kernel-doc

Kernel documentation script complains that some of the function parameters
are not described:

drivers/staging/fbtft/fbtft-core.c:543: warning: Function parameter or member 'pdata' not described in 'fbtft_framebuffer_alloc'

Describe function parameters where it's appropriate.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20191120095716.26628-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>