rt_ssize_t can give negative error code, which follows the unix style correctly

ch569w-evt: add usbhs device mode driver

* usbd driver tested with cdc_vcom, internal loopback
  (can't run both MSH & usbd due to 16KB RAM limitation)
* reduce usrstack & main thread stack size for usbd test
* ch56x_uart.c : iron out UART0_PIN_ALT assignment

add PWM driver, output checked with logic analyzer
spi_xfer() bug fix for cs_pin and message looping
uart pin_mode init moved to uart driver

* ch569w-evt : add spi master driver, SPI0 tested

* Update bsp/wch/risc-v/ch569w-evt/board/Kconfig

* Update bsp/wch/risc-v/ch569w-evt/board/Kconfig
Co-authored-by: Man, Jianting (Meco) <920369182@qq.com>

WCH CH569W-R0-1v0 evt board bsp port, first version

dev/test under Ubuntu 20.04
toolchain from MounRiver_Studio_Community_Linux_x64_V120

tested drivers : SysTick, gpio, gpio interrupt, uart1 (RX interrupt, TX polling)

libcpu/risc-v/SConscript :
group includes rtconfig.CPU only if folder exists

libcpu/risc-v/common/cpuport.c/rt_hw_context_switch_interrupt() :
make it RT_WEAK for customization