UART: Remove ingenic xburst (mips) code from ns8250 driver

Since ingenic JZ4780 SOC support has been removed there is no need
to support ingenic quirks in the UART driver.
Invert of commit b192bae67ea835b7e431225bad375b5d5fe4297f

Reviewed by: imp, manu
Approved by: imp, manu (mentor)
Differential Revision: https://reviews.freebsd.org/D42497