MIPS: JZ4750D: Correct fields for aic/i2s register I2SCR

  In fact bits 1..3 are reserved, remove any macros that touch them.
  Also the ESCLK field (bit 4) was missing for some reason which enables SYSCLK for internal jz4750 codec.
  Add macros __i2s_enable_sysclk() and __i2s_disable_sysclk()