SH4: SCI improvement

This patch simply implement one register of SH4's SCI := Serial Communication Interface.

R2D evaluation board uses SCI for SPI connection.  So, Linux kernel for R2D with
default configuration causes a QEMU assertion failure when it initializes SPI driver.
This patch avoids it and reduces the kernel config modification work for QEMU.

Completing SCI implementation task is left.  Other board support is desirable to confirm
this task, which uses SCI for a serial terminal.

(Shin-ichiro KAWASAKI)

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5939 c046a42c-6fe2-441c-8c8c-71466251a162
