sparc: Fix build warnings in serial.c

These macros are already defined in io.h so should not be declared in
serial.c.

serial.c:38:0: warning: "READ_BYTE" redefined
/home/sjg/c/src/third_party/u-boot/files/include/asm/io.h:36:0: note: this is the location of the previous definition
serial.c:39:0: warning: "READ_HWORD" redefined
/home/sjg/c/src/third_party/u-boot/files/include/asm/io.h:37:0: note: this is the location of the previous definition
serial.c:40:0: warning: "READ_WORD" redefined
/home/sjg/c/src/third_party/u-boot/files/include/asm/io.h:38:0: note: this is the location of the previous definition
serial.c:41:0: warning: "READ_DWORD" redefined
/home/sjg/c/src/third_party/u-boot/files/include/asm/io.h:39:0: note: this is the location of the previous definition

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>